[CI] Publish draft release redux

[s3krit]

This PR is an overhaul of the way we publish draft releases on pushing of a new tag.
The old method was a very cumbersome 274-line bash script in addition to some helper functions in lib.sh.

The new version:

• Outputs releases in exactly the same format as the bash version
• Uses a Github action as opposed to a gitlab-ci job
• Does all of the heavy-lifting in a ruby gem I wrote (which you don't need to worry about in the context of this PR)
• Uses a template for formatting the release notes, which makes it much easier to edit (and separates formatting from logic)
• Accesses Github way less so ends up running faster for releases with lots of changes

Using a real language rather than bash makes it a lot more readable and maintainable, even if you don't know so much Ruby (compared to the weird bash I was having to use).

[kirushik]

It says "Set up Ruby 2.7" just a few lines above

[dvdplm]

Maybe

polkadot_runtime = File.open(filepath) do |f|
  f.find {|l| l =~ /spec_version/ }.match(/[0-9]/+).first
end

…to make sure you close the file after reading?

[s3krit]

Good shout on the file handling :) thanks.
Though for whatever reason, MatchData doesn't have first or last methods...