fix(craft): Add empty Unreleased entry to the changelog after release

[iker-barriocanal]

After updating the changelog entry policy to auto (see #1520), Craft renames the Unreleased entry to the actual version being released. This PR adds the post-release.sh script, run after Craft has finished the release (see docs), creates a new empty entry in the changelog, for future commits.

#skip-changelog

[codecov-commenter]

Codecov Report

Merging #1582 (69c7f03) into main (a85bfc7) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main    #1582   +/-   ##
=========================================
  Coverage     76.02%   76.02%           
  Complexity     1974     1974           
=========================================
  Files           198      198           
  Lines          6837     6837           
  Branches        680      680           
=========================================
  Hits           5198     5198           
  Misses         1306     1306           
  Partials        333      333           

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