[stable5.0] ci: Add release automation

[ChristophWurst]

• Add a fully automated workflow for releases. It does the same as a manual process, except for any milestone cleanups.
• Change the old workflow to only run manually to avoid a double release with the new workflow pushing over to the releases org
• Set up the repository release environment and add necessary keys

This workflow released Mail v4.0.2 yesterday.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 15.43%. Comparing base (138c2ea) to head (35e25e4).
Report is 4 commits behind head on stable5.0.

Additional details and impacted files

@@            Coverage Diff             @@
##           stable5.0    #6507   +/-   ##
==========================================
  Coverage      15.43%   15.43%           
==========================================
  Files            206      206           
  Lines           9441     9441           
  Branches        2193     2204   +11     
==========================================
  Hits            1457     1457           
+ Misses          7666     7665    -1     
- Partials         318      319    +1     

Flag	Coverage Δ
javascript	15.43% <ø> (ø)

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[ChristophWurst]

Reviewers unavailable right now and a release is due -> merge without review