Disallow native passwords for MySQL

[adetunjii]

Description

Resolves the issue causing MySQL backend's registry_test to fail; alternate MySQL errors

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

All modified and coverable lines are covered by tests ✅

Comparison is base (74d7272) 75.19% compared to head (3970ccb) 74.02%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3937      +/-   ##
==========================================
- Coverage   75.19%   74.02%   -1.18%     
==========================================
  Files         330      330              
  Lines       25919    25920       +1     
==========================================
- Hits        19489    19186     -303     
- Misses       5260     5590     +330     
+ Partials     1170     1144      -26     

Files	Coverage Δ
internal/backends/mysql/metadata/pool/uri.go	76.92% <100.00%> (+0.92%)	⬆️

... and 16 files with indirect coverage changes

Flag	Coverage Δ
filter-false	?
filter-true	70.65% <ø> (-1.51%)	⬇️
hana-1	?
integration	70.65% <ø> (-1.53%)	⬇️
mongodb-1	5.17% <ø> (ø)
mysql-1	?
postgresql-1	50.87% <ø> (-12.12%)	⬇️
postgresql-2	50.82% <ø> (ø)
postgresql-3	51.02% <ø> (-0.11%)	⬇️
sqlite-1	50.02% <ø> (-12.78%)	⬇️
sqlite-2	49.83% <ø> (ø)
sqlite-3	50.20% <ø> (-0.11%)	⬇️
unit	30.72% <100.00%> (+<0.01%)	⬆️

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