Refactor Promote Milestone Modal

What does this MR do?

This refactors the "Promote Milestone Modal", replacing DeprecatedModal2 with GlModal. The way the modal is triggered, is changed from an event-bus based solution to one which is "pure" Vue, simplifying the code quite a bit. We moved most of the logic from the init function over to the modal itself. Tests have been adjusted.

Screenshots (strongly suggested)

Before	After
milestone-before	milestone-after

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 #229702 (closed)