Release plugin version 3.0.0

[SethTisue]

As per discussion on #138, the next version is 3.0.0, because newer MiMa and because no more sbt-travisci

I'll start with one or more RCs, which I'll then test in a downstream repo or two.

We've switched from Travis-CI to GitHub Actions in this repo itself, so we're testing that, too.

[SethTisue]

tagged 3.0.0-RC1 but it seems I got something wrong, https://github.com/scala/sbt-scala-module/runs/3475141896 has

[info] gpg: error checking usability status of FFFFFFFF
[info] gpg: key FFFFFFFF: secret key without public key - skipped
[info] gpg: no default secret key: secret key not available
[info] gpg: signing failed: secret key not available
[error] java.lang.RuntimeException: Failure running 'gpg --batch --passphrase *** --detach-sign --armor --use-agent --output /home/runner/work/sbt-scala-module/sbt-scala-module/target/scala-2.12/sbt-1.0/sbt-scala-module-3.0.0-RC1.pom.asc /home/runner/work/sbt-scala-module/sbt-scala-module/target/scala-2.12/sbt-1.0/sbt-scala-module-3.0.0-RC1.pom'.  Exit code: 2

[SethTisue]

c4a7634 and dcb9208 seem to have fixed it; https://github.com/scala/sbt-scala-module/actions/runs/1186933937 published successfully

now to actually try it out downstream

[SethTisue]

I tried 3.0.0-RC3 out in some downstream repos and that went fine, so I went ahead and re-released it as 3.0.0