Fix type inference for `TransactionMigrate::migrate_ok` and friends

[LHolten]

These methods should be parameterized by the return type of the method instead of the return type of the closure.
That will allow removing the closure argument type annotation.

[LHolten]

This may be fine to do with a patch release, need to test on all known uses of rust-query

[LHolten]

TransactionMigrate::migrate_optional returns unit, so it can not work with the solution proposed.