fix: error on postgresql 14 due to BIGSERIAL (#38040)

This pull request makes a targeted update to how PostgreSQL database
types are specified in the courseware/fields.py file. Specifically,
it changes the type returned for PostgreSQL from BIGSERIAL to BIGINT
in both the db_type and rel_db_type methods. This ensures that
auto-incrementing behavior is not implicitly assumed and aligns with 
scenarios where a plain integer type is needed.

lms/djangoapps/courseware/fields.py

@@ -21,7 +21,7 @@ class UnsignedBigIntAutoField(AutoField):
         elif "postgresql" in connection.settings_dict['ENGINE']:
             # Pg's bigserial is implicitly unsigned (doesn't allow negative numbers) and
             # goes 1-9.2x10^18
-            return "BIGSERIAL"
+            return "BIGINT"
         else:
             return None
 
@@ -31,6 +31,6 @@ class UnsignedBigIntAutoField(AutoField):
         elif connection.settings_dict['ENGINE'] == 'django.db.backends.sqlite3':
             return "integer"
         elif "postgresql" in connection.settings_dict['ENGINE']:
-            return "BIGSERIAL"
+            return "BIGINT"
         else:
             return None