[formrecognizer] Unskip v2.0 compatibility tests

[catalinaperalta]

Unskip these tests when the service side issue for listing v2.x models has been resolved.

[kristapratico]

Also unskip sample_manage_custom_models.py and sample_manage_custom_models_async.py from running in CI here: https://github.com/Azure/azure-sdk-for-python/blob/main/scripts/devops_tasks/test_run_samples.py#L128