Add startup warning for debug builds

[AlekSi]

Description

To make things a bit more clear.

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

Merging #3735 (400c7c3) into main (7936b3a) will decrease coverage by 5.69%.
The diff coverage is 0.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3735      +/-   ##
==========================================
- Coverage   76.52%   70.84%   -5.69%     
==========================================
  Files         324      324              
  Lines       23087    23090       +3     
==========================================
- Hits        17667    16357    -1310     
- Misses       4385     5634    +1249     
- Partials     1035     1099      +64     

Files	Coverage Δ
internal/handlers/sqlite/msg_getlog.go	0.00% <0.00%> (-81.66%)	⬇️

... and 57 files with indirect coverage changes

Flag	Coverage Δ
filter-true	66.40% <0.00%> (-6.17%)	⬇️
hana-1	?
integration	66.40% <0.00%> (-6.17%)	⬇️
mongodb-1	5.48% <0.00%> (-0.01%)	⬇️
postgresql-1	51.81% <0.00%> (-0.01%)	⬇️
postgresql-2	51.45% <0.00%> (-0.01%)	⬇️
postgresql-3	?
sqlite-1	51.01% <0.00%> (+0.08%)	⬆️
sqlite-2	50.61% <0.00%> (-0.01%)	⬇️
sqlite-3	?
unit	30.31% <0.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

[rumyantseva]

LGTM