sql: long-running migration to finally remove pre-19.2 foreign keys

[thoszhang]

Currently we're upgrading FKs from the pre-19.2 style representation to the current representation on the fly, not writing them back to disk until we update the table descriptor, so in theory there could still be old-style foreign keys on clusters started in version 19.1 or earlier. We need a long-running migration (#39182) to get rid of these once and for all.

[blathers-crl]

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[thoszhang]

#60510