Refactor project milestone resources

What does this MR do?

This MR refactors the project milestone resources.

The fabricate method is not in use, and so it just adds unnecessary complexity. This is why it is being removed. The same applies to the description attribute.

Also, instead of having a title= method, better having it initialized with a default value so that it doesn't need to be provided when fabricating the resource in the test file. A test that uses one of the updated resources was updated to use the default title.

Also, some duplications were removed.

Finally, this MR reduces the code size in 40 lines.

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