Remove deprecated page and active support depreciation warnings

[theseanything]

The deprecated route (/deprecated) and associated controller method
(ErrorsController#deprecated) are no longer needed since V1 API
usage has been removed in PR #993

This also switches off the active support deprecation warnings features as it's not being used.

[lfdebrux]

@theseanything I think there's some other code from #942 still lying around, including config/initializers/forms_api_deprecations.rb.

[theseanything]

@theseanything I think there's some other code from #942 still lying around, including config/initializers/forms_api_deprecations.rb.

Removed that and the added active support deprecation warnings config.

[github-actions]

🎉 A review copy of this PR has been deployed! It is made of up two components

1. A review copy of forms-runner
2. A production copy of forms-admin

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud