Cleanup pushdown terminology

[noisersup]

Description

Closes #3688.

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 #3691 (8416137) into main (23103e2) will decrease coverage by 5.79%.
The diff coverage is 93.75%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3691      +/-   ##
==========================================
- Coverage   76.53%   70.75%   -5.79%     
==========================================
  Files         323      323              
  Lines       23069    23070       +1     
==========================================
- Hits        17657    16324    -1333     
- Misses       4377     5641    +1264     
- Partials     1035     1105      +70     

Files	Coverage Δ
internal/backends/collection.go	100.00% <ø> (ø)
internal/backends/postgresql/collection.go	72.55% <100.00%> (ø)
internal/backends/sqlite/collection.go	80.26% <100.00%> (ø)
internal/handlers/sqlite/msg_explain.go	58.69% <100.00%> (ø)
internal/handlers/sqlite/msg_aggregate.go	73.24% <0.00%> (-10.77%)	⬇️

... and 58 files with indirect coverage changes

Flag	Coverage Δ
filter-true	66.41% <93.75%> (-6.27%)	⬇️
hana-1	?
integration	66.41% <93.75%> (-6.27%)	⬇️
mongodb-1	5.45% <0.00%> (-0.01%)	⬇️
postgresql-1	51.85% <37.50%> (+0.02%)	⬆️
postgresql-2	51.46% <31.25%> (-0.05%)	⬇️
postgresql-3	?
sort-false	66.41% <93.75%> (-6.27%)	⬇️
sqlite-1	50.94% <75.00%> (-0.05%)	⬇️
sqlite-2	50.62% <68.75%> (-0.05%)	⬇️
sqlite-3	?
unit	30.17% <62.50%> (-0.01%)	⬇️

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

[rumyantseva]

I don't have my own questions about the PR (I agree with what Alexey asked).

[rumyantseva]

LGTM