migration: Make 'rename_indexes_constraints' a lib function.

This prep commit moves the 'rename_indexes_constraints'
function to 'lib/migrate' as we're going to re-use it for
the 'UserHotspot' to 'OnboardingStep' table rename operation.

In general, this function would be helpful in migrations
involving table rename operations, subject to the caution
mentioned in the function via comments.

tools/semgrep-py.yml

@@ -68,6 +68,7 @@ rules:
         - zerver/migrations/0209_user_profile_no_empty_password.py
         - zerver/migrations/0260_missed_message_addresses_from_redis_to_db.py
         - zerver/migrations/0387_reupload_realmemoji_again.py
+        - zerver/migrations/0443_userpresence_new_table_schema.py
         - pgroonga/migrations/0002_html_escape_subject.py
 
   - id: html-format