"Allow milestones to be associated with releases" BE

What does this MR do?

This MR creates a many-to-many relationship between Milestone and Release. In #62402 (moved), even though the MVC for this new feature will only allow one milestone for one release, it was decided to go ahead and implement the long-term architecture.

Also, this MR updates accordingly the Release API, by modifying the create and update release endpoints so one Milestone can be added/removed on API call (given the scope of the MVC). The API documentation has also been updated.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content - not relevant here.

Performance 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 This MR is about changing backend. However, releases and milestones were created from the UI on: Chrome, Firefox

Security

Section not relevant to this MR.

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 #62402 (moved)