Openapi `Purge` schema has invalid date-time format

Version
pulpcore: 3.23.0
galaxy_ng: 4.7.0

Describe the bug
default date-time format is invalid and generated openapi Purge schema fails on validate_spec

=================================== FAILURES ===================================
________________________ test_galaxy_openapi_validation ________________________
galaxy_ng/tests/integration/api/test_openapi.py:65: in test_galaxy_openapi_validation
    validate_spec(galaxy_spec)
/tmp/gng_testing/lib/python3.10/site-packages/openapi_spec_validator/shortcuts.py:17: in validate_spec
    return validator.validate(spec, spec_url=spec_url)
/tmp/gng_testing/lib/python3.10/site-packages/openapi_spec_validator/validation/proxies.py:28: in validate
    raise err
E   openapi_spec_validator.validation.exceptions.OpenAPIValidationError: '2023-01-14' is not a 'date-time'
E   
E   Failed validating 'format' in schema:
E       {'default': '2023-01-14',
E        'description': 'Purge tasks completed earlier than this timestamp. '
E                       "Format '%Y-%m-%d[T%H:%M:%S]'",
E        'format': 'date-time',
E        'type': 'string'}
E   
E   On instance:
E       '2023-01-14'
=========================== short test summary info ============================
FAILED galaxy_ng/tests/integration/api/test_openapi.py::test_galaxy_openapi_validation - openapi_spec_validator.validation.exceptions.OpenAPIValidationError: '2023-01-14' is not a 'date-time'

To Reproduce
Run the galaxy_ng integration test (https://github.com/ansible/galaxy_ng/blob/2553ee6c136db83c2afb306d2fbea6879cf9c870/galaxy_ng/tests/integration/api/test_openapi.py#L57) or run validate_spec on openapi.json

Start with galaxy_ng/tests/integration/api/test_openapi.py at the validation call referenced in the issue, then inspect the generated openapi.json and the Purge schema. Use validate_spec to reproduce the failure; done means the generated Purge default conforms to the declared date-time format and the integration validation passes.