Remove fjson package

[AlekSi]

Description

Closes #4157.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 73.84%. Comparing base (34a6fa5) to head (9d2e43b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4176      +/-   ##
==========================================
- Coverage   73.85%   73.84%   -0.02%     
==========================================
  Files         353      339      -14     
  Lines       22985    22785     -200     
==========================================
- Hits        16975    16825     -150     
+ Misses       4710     4676      -34     
+ Partials     1300     1284      -16     

Files	Coverage Δ
internal/types/types.go	93.54% <ø> (ø)

... and 5 files with indirect coverage changes

Flag	Coverage Δ
filter-true	67.29% <ø> (-0.01%)	⬇️
integration	67.29% <ø> (-0.01%)	⬇️
mongodb-1	4.89% <ø> (ø)
postgresql-1	46.62% <ø> (+0.03%)	⬆️
postgresql-2	49.45% <ø> (-0.04%)	⬇️
postgresql-3	49.35% <ø> (+0.01%)	⬆️
sqlite-1	45.78% <ø> (+0.02%)	⬆️
sqlite-2	48.60% <ø> (+0.04%)	⬆️
sqlite-3	48.51% <ø> (ø)
unit	32.55% <ø> (-0.39%)	⬇️

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