Added a feature flag for add_srs_entry support; refs #22632 and #23504.
Added feature flag for geometry_field_introspection; refs #22632 and #23504.
Improved GIS migration test cleanup; refs #23504.
Fixed #23488 -- Added AnonymousUser.get_username().
Removed unnecessary order_by() from a GIS test that crashed Oracle.

Oracle cannot order_by() a TextField; refs #23504.
Fixed string length check in LayerMapping
Fixed #23514 -- Prevented queries in PostGISOperations init

Thanks Mattia Procopio for the report.
Fixed #23443 -- Corrected erroneous FieldError message.

Thanks Tim Graham for the review.