Expanded information on deprecations in changelog

[schlunma]

See ESMValGroup/ESMValTool#2704 (comment)

Link to documentation: https://esmvaltool--1658.org.readthedocs.build/projects/ESMValCore/en/1658/changelog.html#deprecations

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[codecov]

Codecov Report

Merging #1658 (19a27ea) into main (aacec50) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1658   +/-   ##
=======================================
  Coverage   91.49%   91.49%           
=======================================
  Files         204      204           
  Lines       11175    11175           
=======================================
  Hits        10225    10225           
  Misses        950      950           

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[schlunma]

Ready for review

[bouweandela]

Thanks for taking care!

[sloosvel]

Thanks!