[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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

```diff @@ 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](https://app.codecov.io/gh/nextcloud/calendar/pull/6507/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | Coverage Δ | | |---|---|---| | [javascript](https://app.codecov.io/gh/nextcloud/calendar/pull/6507/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud) | `15.43% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nextcloud#carryforward-flags-in-the-pull-request-comment) to find out more.

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[ChristophWurst]

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