Rename bson2 to bson

[AlekSi]

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.

[codecov]

Codecov Report

Attention: Patch coverage is 77.50000% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 73.82%. Comparing base (365a3dd) to head (5d062f3).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4179      +/-   ##
==========================================
- Coverage   74.53%   73.82%   -0.72%     
==========================================
  Files         339      339              
  Lines       22786    22786              
==========================================
- Hits        16984    16821     -163     
- Misses       4499     4681     +182     
+ Partials     1303     1284      -19     

Files	Coverage Δ
internal/bson/array.go	65.45% <ø> (ø)
internal/bson/bson.go	83.07% <ø> (ø)
internal/bson/decode.go	93.84% <ø> (ø)
internal/bson/decodemode_string.go	0.00% <ø> (ø)
internal/bson/document.go	61.76% <ø> (ø)
internal/bson/encode.go	58.53% <ø> (ø)
internal/bson/logging.go	90.41% <ø> (ø)
internal/bson/oldbson/array.go	70.45% <ø> (ø)
internal/bson/oldbson/binary.go	62.06% <ø> (ø)
internal/bson/oldbson/bool.go	64.00% <ø> (ø)
... and 21 more

... and 14 files with indirect coverage changes

Flag	Coverage Δ
filter-true	67.27% <52.50%> (-0.82%)	⬇️
hana-1	?
integration	67.27% <52.50%> (-0.82%)	⬇️
mongodb-1	4.89% <0.00%> (ø)
postgresql-1	46.58% <52.50%> (ø)
postgresql-2	49.38% <52.50%> (-0.01%)	⬇️
postgresql-3	49.33% <52.50%> (+0.02%)	⬆️
sqlite-1	45.78% <52.50%> (ø)
sqlite-2	48.54% <52.50%> (-0.11%)	⬇️
sqlite-3	48.54% <52.50%> (ø)
unit	32.54% <70.00%> (ø)

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