Fix collection name starting with dot validation

[noisersup]

Description

Readiness checklist

• I added/updated unit tests.
• I added/updated integration/compatibility tests.
• 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), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #2912 (bd3872b) into main (eb6170c) will increase coverage by 0.05%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2912      +/-   ##
==========================================
+ Coverage   63.61%   63.66%   +0.05%     
==========================================
  Files         451      451              
  Lines       23577    23583       +6     
==========================================
+ Hits        14999    15015      +16     
+ Misses       7610     7602       -8     
+ Partials      968      966       -2     

Impacted Files	Coverage Δ
internal/handlers/pg/msg_create.go	73.75% <100.00%> (+1.02%)	⬆️
internal/handlers/pg/pgdb/collections.go	70.73% <100.00%> (+0.73%)	⬆️

... and 3 files with indirect coverage changes

Flag	Coverage Δ
integration	56.94% <100.00%> (+0.05%)	⬆️
mongodb	4.39% <0.00%> (-0.01%)	⬇️
pg	56.85% <100.00%> (+0.05%)	⬆️
shard-1	41.99% <0.00%> (-0.02%)	⬇️
shard-2	45.30% <100.00%> (+0.06%)	⬆️
shard-3	40.64% <0.00%> (+<0.01%)	⬆️
unit	24.26% <0.00%> (-0.01%)	⬇️

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[w84thesun]

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