Remove CentOS 7 from list of build targets (#6864)

We can no longer build a native CentOS 7 RPM (at least not with the
default toolchain), and it seems like making it work would be harder and
harder.  If you're still running CentOS 7, switch to the generic
portable build instead, which can still run on CentOS 7 and is
supported.

.github/workflows.src/build.targets.yml

@@ -79,12 +79,6 @@ targets:
           platform_version: jammy
           family: debian
           runs_on: [self-hosted, linux, arm64]
-        - name: centos-7-x86_64
-          arch: x86_64
-          platform: centos
-          platform_version: 7
-          family: redhat
-          runs_on: [self-hosted, linux, x64]
         - name: centos-8-x86_64
           arch: x86_64
           platform: centos