Deprecate seurat conversion

[brianraymor]

Context

Starting with the 5.3 schema migration, no new Seurat conversions must be performed. Existing Seurat dataset versions must remain available to data consumers.

See references.

The order of operations for releasing this should be:

1. Deploy a version of data-portal to prod where we stop running seurat conversions as part of CXG conversion and any product changes that we may want to also include. This will require some coordination with someone on the Applications team
2. Ensure that schema 5.3 has this change which drops the seurat convertible return value from cellxgene-schema
3. Run the schema 5.3 migration in all environments
4. Update data-portal to use cellxgene-schema 5.3, which should include this change to drop the is_seurat_convertible return