Add missing transaction commit to SEPA mandate migration

The console does not commit the transaction automatically, so the updated
addresses are not persisted without an explicit commit.

Closes #15037

doc/migration.rst

@@ -111,6 +111,7 @@ After
          mandate.on_change_party()
 
      Mandate.save(mandates)
+     transaction.commit()
 
 .. _migration-7.6: