Cleanup TODO for speculative authenticate

[chilagrow]

Description

Both TODO does not seem to be related to speculative authenticate 🤔, so I created issues for them. Let me know if I'm wrong.

One is about supporting filter in usersInfo command #4141, another is creating user for $external database #4142.

Closes #3777.

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 73.92%. Comparing base (25e6b46) to head (c358160).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4143      +/-   ##
==========================================
- Coverage   74.89%   73.92%   -0.98%     
==========================================
  Files         353      353              
  Lines       22680    22680              
==========================================
- Hits        16987    16767     -220     
- Misses       4413     4638     +225     
+ Partials     1280     1275       -5     

Files	Coverage Δ
internal/handler/msg_createuser.go	75.52% <ø> (ø)
internal/handler/msg_usersinfo.go	76.72% <ø> (ø)

... and 30 files with indirect coverage changes

Flag	Coverage Δ
filter-false	?
filter-true	67.24% <ø> (-1.39%)	⬇️
hana-1	?
integration	67.24% <ø> (-1.46%)	⬇️
mongodb-1	4.75% <ø> (ø)
mysql-1	?
mysql-2	?
mysql-3	?
postgresql-1	48.27% <ø> (-0.10%)	⬇️
postgresql-2	?
postgresql-3	49.22% <ø> (-0.10%)	⬇️
sqlite-1	47.49% <ø> (-0.10%)	⬇️
sqlite-2	47.91% <ø> (-0.05%)	⬇️
sqlite-3	48.26% <ø> (-0.15%)	⬇️
unit	32.65% <ø> (ø)

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