Rename `approval_status` to `classification`

What does this MR do?

This MR renames the software_license_policies.approval_status column to software_license_policies.classification to align with the naming defined in #14061 (closed). This change does not change the existing JSON attributes or params keys sent by user agents. A subsequent MR can be produced that aligns the front end code with the back end.

#36782 (closed)

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