Adds the 1.2.1 CHANGELOG section to master

[carmocca]

What does this PR do?

See title
Related to #6154

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[codecov]

Codecov Report

Merging #6156 (7327965) into master (ebabe56) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #6156   +/-   ##
======================================
- Coverage      93%     93%   -0%     
======================================
  Files         160     160           
  Lines       11435   11435           
======================================
- Hits        10690   10655   -35     
- Misses        745     780   +35