Use uniform release headers in CHANGES.md

[fingolfin]

This starts each release entry with a uniform header, ## VERSION (YYYY-MM-DD). We are moving the changelogs of the packages in gap-packages to this format, so that scripts can extract the notes of each release.

Dates missing from the file, or given only as a month, are taken from the git history: the date in PackageInfo.g at the release tag, or the date of the tagged commit. Where neither exists, the entry keeps its month (YYYY-MM) or has no date. Otherwise only headers and formatting change: indentation, blank lines, separator lines and list markers.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.73%. Comparing base (c93e2a0) to head (65ac658).

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #27   +/-   ##
=======================================
  Coverage   91.73%   91.73%           
=======================================
  Files          15       15           
  Lines        1537     1537           
=======================================
  Hits         1410     1410           
  Misses        127      127           

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