feat(system/roles): add Deprecation header on old path

GET /auth/system/roles is superseded by GET /auth/resource/system/roles,
which has been live since the system Blueprint was registered under the
resources Blueprint.  Add Deprecation: true and a Link successor-version
header when the endpoint is reached via the old path, so callers can
discover the new URL.

The old registration in auth/views.py already carries a TODO noting it
should be removed once consumers are updated.  Callers to migrate:
- GN2: wqflask/views.py, metadata_edits.py (×3), flask_extensions.py,
        oauth2/ui.py, oauth2/request_utils.py, oauth2/resources.py (×2)
- GN3: gn3/api/metadata.py, gn3/api/case_attributes.py
- gn-integration-tests: tests/test_gn_auth_smoke.py

Reviewed-By: Frederick M. Muriithi <fredmanglis@gmail.com>

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