Add blog post for FerretDB v1.21 release

[Fashander]

Description

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.

[mergify]

Marketing blog posts should be reviewed by @ptrfarkas and @AlekSi.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 33.61%. Comparing base (a3fb274) to head (042fead).

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4202       +/-   ##
===========================================
- Coverage   75.05%   33.61%   -41.44%     
===========================================
  Files         339      339               
  Lines       22876    22876               
===========================================
- Hits        17169     7690     -9479     
- Misses       4412    14378     +9966     
+ Partials     1295      808      -487     

see 194 files with indirect coverage changes

Flag	Coverage Δ
filter-false	?
filter-true	5.13% <ø> (-63.70%)	⬇️
hana-1	?
integration	5.13% <ø> (-63.81%)	⬇️
mongodb-1	5.13% <ø> (-0.01%)	⬇️
mysql-1	?
mysql-2	?
mysql-3	?
postgresql-1	?
postgresql-2	?
postgresql-3	?
sqlite-1	?
sqlite-2	?
sqlite-3	?
unit	32.52% <ø> (ø)

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