Replace pgxtype.Querier with pgx.Tx

[w84thesun]

Description

This PR closes #1161.

Readiness checklist

• I added tests for new functionality or bugfixes.
• I ran task all, and it passed.
• I added/updated comments for both exported and unexported top-level declarations (functions, types, etc).
• I checked comments rendering with task godocs.
• I ensured that the title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Assignee, Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[vercel]

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Name	Status	Preview	Updated
ferret-db	✅ Ready (Inspect)	Visit Preview	Sep 28, 2022 at 2:10PM (UTC)

[codecov]

Codecov Report

Merging #1188 (45f3bdd) into main (e919083) will decrease coverage by 0.08%.
The diff coverage is 86.95%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1188      +/-   ##
==========================================
- Coverage   66.41%   66.33%   -0.09%     
==========================================
  Files         251      251              
  Lines       12293    12293              
==========================================
- Hits         8165     8154      -11     
- Misses       3238     3248      +10     
- Partials      890      891       +1     

Impacted Files	Coverage Δ
internal/handlers/pg/pgdb/query.go	40.68% <50.00%> (ø)
internal/handlers/pg/pgdb/settings.go	39.53% <82.35%> (-4.66%)	⬇️
internal/handlers/pg/msg_listcollections.go	61.11% <100.00%> (ø)
internal/handlers/pg/pgdb/collections.go	50.00% <100.00%> (ø)
internal/handlers/pg/pgdb/schemas.go	80.00% <100.00%> (ø)
internal/handlers/pg/pgdb/tables.go	64.70% <100.00%> (ø)
internal/clientconn/conninfo/conn_info.go	66.66% <0.00%> (-16.67%)	⬇️
internal/util/version/version.go	55.00% <0.00%> (-5.00%)	⬇️
internal/handlers/common/filter.go	86.58% <0.00%> (ø)

Flag	Coverage Δ
integration	62.62% <86.95%> (-0.08%)	⬇️
mongodb	14.87% <8.69%> (+0.01%)	⬆️
pg	51.86% <86.95%> (-0.08%)	⬇️
tigris	34.67% <0.00%> (-0.03%)	⬇️
unit	24.00% <80.43%> (-0.05%)	⬇️

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[rumyantseva]

I'm not 100% sure about pgdb_test.go (do we want everything to have their own transaction?)

But in general, LGTM.

[rumyantseva]

LGTM