Continue offering non-CI/GHA versions of plugins

[armanbilge]

This one comes from @bpholt, to avoid issues with sbt-project-matrix which doesn't play nice with sbt-gh-actions. The current setup caters perfectly fine for this use-case (e.g., the separate sbt-typelevel-sonatype and sbt-typelevel-sonatype-ci-release plugins. It's a minor tax on this repo, but much more friendly to downstream.

[armanbilge]

I'm not sure how important this is anymore, thanks to some developments.

• I'm hopeful that Introduce sbtStepPreamble to GHA plugin #64 will fix the sbt-projectmatrix interop
• Even Rob was won over by sbt-gh-actions 😝

[armanbilge]

I like the separation of concerns, but I fully admit the entire repo's organization could be re-evaluated. But that is broader than this issue.