Prepare release 2.1.0

[artemgavrilov]

This PR provides preparations for releasing 2.1.0:

1. Updates META.json according to spec
2. Updates meson.build
3. Removes redundant RELEASE_NOTES.md and mkdocs.yml

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.87%. Comparing base (3bb6579) to head (dd6bc6c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #482   +/-   ##
=======================================
  Coverage   84.87%   84.87%           
=======================================
  Files           3        3           
  Lines        1296     1296           
  Branches      198      198           
=======================================
  Hits         1100     1100           
  Misses         98       98           
  Partials       98       98           

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