Part 2 of project milestone resource refactoring

What does this MR do?

This MR moves attributes of a project milestone resource to the main class so that the EE one can inherit it, and focus only on EE specific stuff.

And one of the attributes is being removed since it wasn't in use.

Also, it prefers to be explicit about how fabrication happens (via API) than being implicit (fabricate!)

Finally, this change saves us six lines of code.

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