Downgrade CREATE_INDEX_EXCLUSIVE to a warning

While it's always a dangerous *pattern* to create an index while a
transaction is open & holds an exclusive lock, in practice it's not
always a big deal (for example, indexing a table with very few rows can
be fast). Accordingly, downgrade to a warning.

django_migration_linter/sql_analyser/postgresql.py

@@ -67,7 +67,7 @@ class PostgresqlAnalyser(BaseAnalyser):
             fn=has_create_index_in_transaction,
             message="CREATE INDEX prolongs transaction, delaying lock release",
             mode=CheckMode.TRANSACTION,
-            type=CheckType.ERROR,
+            type=CheckType.WARNING,
         ),
         Check(
             code="DROP_INDEX",