Prepare v1.21.0 release

[AlekSi]

Description

Plus tweak telemetry logging a bit.

Readiness checklist

• I added/updated unit tests (and they pass).
• I added/updated integration/compatibility tests (and they pass).
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Milestone (Next), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 73.92%. Comparing base (60fcd09) to head (1153fbf).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4195      +/-   ##
==========================================
- Coverage   75.02%   73.92%   -1.10%     
==========================================
  Files         339      339              
  Lines       22876    22876              
==========================================
- Hits        17162    16912     -250     
- Misses       4416     4681     +265     
+ Partials     1298     1283      -15     

Files	Coverage Δ
internal/util/state/state.go	100.00% <100.00%> (ø)
internal/util/telemetry/reporter.go	46.56% <100.00%> (+1.45%)	⬆️
internal/handler/msg_getlog.go	83.15% <0.00%> (ø)

... and 31 files with indirect coverage changes

Flag	Coverage Δ
filter-false	?
filter-true	67.40% <25.00%> (-1.45%)	⬇️
hana-1	?
integration	67.40% <25.00%> (-1.52%)	⬇️
mongodb-1	5.13% <0.00%> (-0.01%)	⬇️
mysql-1	?
mysql-2	?
mysql-3	?
postgresql-1	46.54% <25.00%> (-0.03%)	⬇️
postgresql-2	49.61% <12.50%> (-0.11%)	⬇️
postgresql-3	49.84% <25.00%> (-0.06%)	⬇️
sqlite-1	45.63% <25.00%> (-0.15%)	⬇️
sqlite-2	48.78% <12.50%> (-0.06%)	⬇️
sqlite-3	49.00% <25.00%> (-0.06%)	⬇️
unit	32.52% <57.14%> (+0.01%)	⬆️

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