Enhance GH action to publish to RubyGems automatically on version changes

[megatux]

See for instance this another RS action

[guillermoap]

Only comment here is that I see that you require ruby '>= 2.3.0' and have 2.7 in the CI test suite so you should use v2 of the action, which is the one that uses ruby 2

- uses: discourse/publish-rubygems-action@v3
+ uses: discourse/publish-rubygems-action@v2