Automate publishing of release

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Invoked via bb publish. Read new doc/maintainer-guide.adoc for details.

Other changes of note:

• moved to GitHub Actions for CI
• added CI test matrix for jdk 8, 11, 17 and ubuntu, windows
• added bb tasks
• completed transition from lein to deps.edn + build.clj

Details of interest

• adjust jdk 8 javac compatibility for jdk >= v9
• using neil convention of storing version in deps.edn
• tagging with v1.2.3 instead of Release-1.2.3
• now at version 1.x (was 0.x)
• version patch is now release count, next ver should be 1.0.26 (was commit count)
  • clj-yaml - 3 releases
  • circleci/clj-yaml - 11 releases
  • clj-commons/clj-yaml - 11 releases
• no longer gpg signing jars, we don't understand the value and don't want the complexity

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Todo:

• [x] stop building on circleci (hey there might be other changes coming in while we review this, yeah? I'll do this after the merge)
• [x] configuring scoped clojars tokens as publish environment secrets in this repo (I'll report back when this done).

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I'll slap the project.clj back in temporarily to get rid of the circleci red. Tomorrow. That's enough for today.

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TIL: I guess the GitHub actions feature cannot be activated from a PR? Creating a dummy action workflow on the master branch did the trick.