Deprecate additions and deletions attributes in Repositories API

What does this MR do?

This marks the additions and deletions attributes as deprecated in the Repositories API, because they always return zero. This is the first step in resolving #233119 (closed). The second step is removing the attributes in !38920 (closed) (which will be completed after the deprecation period).

The deprecation notice in this MR is modeled after a deprecation notice in the Issues API.

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 #233119 (closed)