Rename `approved` to `allowed`

What does this MR do?

This merge request renames approved to allowed as per the new terms discussed and committed to in:

• #14061 (closed)
• #12937 (comment 211525928)

The existing managed licenses API endpoint will continue to behave the old way. It still serves the old values using the old attribute name.

New code being written today for the new License Compliance page will be able to use the new terms.

I did not add a changelog entry as this is a backstage change that does not change any end user facing text of functionality. Subsequent front end MR's will follow this to address documentation and text in the UI.

This MR is a continuation of the work started in !21261 (merged)

#38278 (closed)

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

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