Drop support for license-management artifacts

What does this MR do?

Issue #216995 (closed)

We need to drop support for license_management. It can be done by not allowing to save license-management artifact. That forces users to switch to new license_scanning artifacts (if they haven't done it already in 12.8).

Tha change is behind feature flag drop_license_management_artifact.

What is in this MR?

• Update artifact validation, so it doesn't create license_management artifacts
• Fixes of tests to workaround validations: use feature flag to validate license_management artifacts, replace when is possible with license_scanning artifacts.
• Update docs

Screenshots

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