fix: fix hydra_client pk change mysql down migration

[grantzvolsky]

Related issue(s)

#2790
@aeneasr

Checklist

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• I am following the
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• I have read the security policy.
• I confirm that this pull request does not address a security
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• [] I have added tests that prove my fix is effective or that my feature
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• [n/a] I have added or changed the documentation.

Further Comments

I tested the change manually by writing the output of show create table hydra_client; before the up migration (a), after the up migration (b), and after the down migration (c), and making sure that a == c. We might want to add a test that does this automatically for all down migrations, or at least a convenience script that runs this procedure for a given migration.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (v2.x@09ac91b). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             v2.x    #2791   +/-   ##
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  Coverage        ?   52.85%           
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  Branches        ?        0           
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  Hits            ?     7498           
  Misses          ?     6054           
  Partials        ?      633           

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