Add API endpoint /application/plan_limits for package file size limits

What does this MR do?

In the UI, the "Package file size limits" can be defined in the application settings (Settings > CI/CD > Package Registry):

However, these settings are not accessible via the API.

In this MR, the following package file size settings of the default plan are integrated into the API endpoint GET/PUT /application/settings (see current documentation). According to the discussion in this MR, it's not desired that the settings are included into the API endpoint /application/settings. Instead, it was requested that the plan limits should be outsourced to a new, separate API endpoint.

So in this MR, a new API endpoint /application/plan_limits is implemented to allow you to maintain the application limits for the existing subscription plans. The plan limits can now be read and modified directly through the API.

Category:API api backend

🛠 with ❤ at Siemens

/cc @bufferoverflow

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation
• 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