Disallow $natural in compound sort

[noisersup]

Description

Closes #3638.

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 #3832 (c402fdc) into main (26b27de) will decrease coverage by 0.13%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3832      +/-   ##
==========================================
- Coverage   76.30%   76.18%   -0.13%     
==========================================
  Files         316      316              
  Lines       23655    23662       +7     
==========================================
- Hits        18051    18027      -24     
- Misses       4560     4588      +28     
- Partials     1044     1047       +3     

Files	Coverage Δ
internal/handler/common/sort.go	85.80% <100.00%> (+0.64%)	⬆️

... and 13 files with indirect coverage changes

Flag	Coverage Δ
filter-true	72.34% <100.00%> (-0.13%)	⬇️
hana-1	?
integration	72.34% <100.00%> (-0.13%)	⬇️
mongodb-1	5.18% <0.00%> (-0.01%)	⬇️
postgresql-1	52.08% <100.00%> (+0.09%)	⬆️
postgresql-2	52.25% <0.00%> (-0.07%)	⬇️
postgresql-3	52.13% <0.00%> (-0.05%)	⬇️
sqlite-1	51.35% <100.00%> (+0.04%)	⬆️
sqlite-2	51.38% <0.00%> (-0.02%)	⬇️
sqlite-3	51.25% <0.00%> (-0.03%)	⬇️
unit	30.64% <0.00%> (-0.01%)	⬇️

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[rumyantseva]