Remove CentOS 7 tests

[dralley]

CentOS 7 is EOL, and using the vault URLs is somewhat impolite as they aren't cached.

[noissue]

[dralley]

Filed #3656 to handle re-enabling some of these tests that no longer have fixtures.

[patchback]

Backport to 3.18: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 99d657d on top of patchback/backports/3.18/99d657d53153fc698c64bfb3d35c558574438092/pr-3655

Backporting merged PR #3655 into main

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/pulp/pulp_rpm.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.18/99d657d53153fc698c64bfb3d35c558574438092/pr-3655 upstream/3.18

4. Now, cherry-pick PR Remove CentOS 7 tests #3655 contents into that branch:

   $ git cherry-pick -x 99d657d53153fc698c64bfb3d35c558574438092

   If it'll yell at you with something like fatal: Commit 99d657d53153fc698c64bfb3d35c558574438092 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 99d657d53153fc698c64bfb3d35c558574438092

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Remove CentOS 7 tests #3655 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.18/99d657d53153fc698c64bfb3d35c558574438092/pr-3655

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.19: 💔 cherry-picking failed — conflicts found

❌ Failed to cleanly apply 99d657d on top of patchback/backports/3.19/99d657d53153fc698c64bfb3d35c558574438092/pr-3655

Backporting merged PR #3655 into main

1. Ensure you have a local repo clone of your fork. Unless you cloned it
   from the upstream, this would be your origin remote.
2. Make sure you have an upstream repo added as a remote too. In these
   instructions you'll refer to it by the name upstream. If you don't
   have it, here's how you can add it:

   $ git remote add upstream https://github.com/pulp/pulp_rpm.git

3. Ensure you have the latest copy of upstream and prepare a branch
   that will hold the backported code:

   $ git fetch upstream
   $ git checkout -b patchback/backports/3.19/99d657d53153fc698c64bfb3d35c558574438092/pr-3655 upstream/3.19

4. Now, cherry-pick PR Remove CentOS 7 tests #3655 contents into that branch:

   $ git cherry-pick -x 99d657d53153fc698c64bfb3d35c558574438092

   If it'll yell at you with something like fatal: Commit 99d657d53153fc698c64bfb3d35c558574438092 is a merge but no -m option was given., add -m 1 as follows instead:

   $ git cherry-pick -m1 -x 99d657d53153fc698c64bfb3d35c558574438092

5. At this point, you'll probably encounter some merge conflicts. You must
   resolve them in to preserve the patch from PR Remove CentOS 7 tests #3655 as close to the
   original as possible.
6. Push this branch to your fork on GitHub:

   $ git push origin patchback/backports/3.19/99d657d53153fc698c64bfb3d35c558574438092/pr-3655

7. Create a PR, ensure that the CI is green. If it's not — update it so that
   the tests and any other checks pass. This is it!
   Now relax and wait for the maintainers to process your pull request
   when they have some cycles to do reviews. Don't worry — they'll tell you if
   any improvements are necessary when the time comes!

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.22: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.22/99d657d53153fc698c64bfb3d35c558574438092/pr-3655

Backported as #3658

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.23: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.23/99d657d53153fc698c64bfb3d35c558574438092/pr-3655

Backported as #3659

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.25: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.25/99d657d53153fc698c64bfb3d35c558574438092/pr-3655

Backported as #3660

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.27: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.27/99d657d53153fc698c64bfb3d35c558574438092/pr-3655

Backported as #3661

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.

[patchback]

Backport to 3.26: 💚 backport PR created

✅ Backport PR branch: patchback/backports/3.26/99d657d53153fc698c64bfb3d35c558574438092/pr-3655

Backported as #3662

🤖 @patchback
I'm built with octomachinery and
my source is open — https://github.com/sanitizers/patchback-github-app.