fix: handle constraints in db migration for MySQL 8

[exemplary-citizen]

SUMMARY
fix for db migration issue introduced in #16756

TESTING INSTRUCTIONS
Watch CI and any suggestions from reviewers

cc: @betodealmeida

[codecov]

Codecov Report

Merging #17274 (518e7ad) into master (ce558e0) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #17274      +/-   ##
==========================================
- Coverage   77.02%   76.95%   -0.08%     
==========================================
  Files        1037     1037              
  Lines       55629    55629              
  Branches     7594     7594              
==========================================
- Hits        42851    42807      -44     
- Misses      12528    12572      +44     
  Partials      250      250              

Flag	Coverage Δ
hive	81.52% <ø> (ø)
mysql	81.95% <ø> (ø)
postgres	81.96% <ø> (ø)
presto	?
python	82.31% <ø> (-0.16%)	⬇️
sqlite	81.63% <ø> (ø)

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Impacted Files	Coverage Δ
superset/db_engine_specs/presto.py	84.30% <0.00%> (-6.07%)	⬇️
superset/connectors/sqla/models.py	85.99% <0.00%> (-1.41%)	⬇️
superset/models/core.py	89.26% <0.00%> (-0.74%)	⬇️

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[betodealmeida]

@exemplary-citizen did you test this on an existing install with MySQL 8?

I don't think it will work because the constraint will already be there for existing installations, and for new installations it will be created before this migration runs.

[betodealmeida]

I think we need to do something like this: #17275

[exemplary-citizen]

@betodealmeida you're right it is still failing. closing in favor of your PR