Add milestone filtering support in Roadmap

What does this MR do?

Note: This feature is currently behind feature flag :async_filtering, you can enable it on per-group basis from Rails console by running Feature.enable(:async_filtering, Group.find_by_full_path('gitlab-org')).

This MR adds support for Milestone token in filtered search bar which was added in !36421 (merged).

Screenshots

Tokens List	Token Values	Token Selected

Does this MR meet the acceptance criteria?

Conformity

• 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

Closes #218621 (closed)