Use declarative_policy gem

What does this MR do?

This completes the process of open-sourcing the declarative policy library (#325463 (closed)), by replacing the code from this repository with the extracted gem.

Closes #325463 (closed)

Closes https://gitlab.com/gitlab-org/declarative-policy/-/issues/7

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have not included a changelog entry because it is a developer facing change.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

Tests are removed that have been moved to the gem. Existing policy specs should all pass.

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

This makes a fundamental change to the way we handle authorization. It definitely requires a security review from @gitlab-com/gl-security/appsec

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team