Update database naming restrictions

[b1ron]

Description

We now allow database names to start with numbers.

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), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

Merging #3042 (291d353) into main (4bab468) will decrease coverage by 2.16%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3042      +/-   ##
==========================================
- Coverage   76.42%   74.26%   -2.16%     
==========================================
  Files         386      386              
  Lines       21120    21120              
==========================================
- Hits        16140    15684     -456     
- Misses       4058     4481     +423     
- Partials      922      955      +33     

see 26 files with indirect coverage changes

Flag	Coverage Δ
integration	70.68% <ø> (-2.25%)	⬇️
mongodb	5.41% <ø> (ø)
pg	60.43% <ø> (-5.90%)	⬇️
shard-1	54.20% <ø> (-0.02%)	⬇️
shard-2	36.87% <ø> (-18.85%)	⬇️
shard-3	57.10% <ø> (ø)
sqlite	46.53% <ø> (ø)
unit	24.25% <ø> (ø)

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