Remove not null constraint on framework column

What does this MR do?

This MR removes the NOT NULL constraint on the framework column in the project_compliance_framework_settings table and makes the application code use the recently introduced association: ComplianceManagement::Framework

• Remove the NOT NULL constraint.
• Remove the enum in AR.
• Change the GraphQL enum for the compliance frameworks to String. We will allow custom frameworks.
• Add a new constant DEFAULT_FRAMEWORKS which keeps track of the previously existing "default" frameworks.
  • This is needed for backward compatibility.
  • Application settings (compliance_frameworks column) uses it.
• Update the test to use the compliance_management_framework association instead of the framework enum.

Current status:

This MR:

• framework column still exists, but it's ignored.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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:

• 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

Related to #251113 (closed)