Fix(oncallschedules): update rotation form submission validation

What does this MR do?

An MVC as part of https://gitlab.com/gitlab-org/gitlab/-/issues/322137 that:

1. Removes assignee dropdown CSS in favour of the menu-class prop for the GlTokenSelector.
2. Updates the rotation form validation to stop submissions of the form when the title or assignees field is empty.

How/What to test

This feature is behind the feature flag oncall_schedules_mvc and the minimal license is GitLab Premium. You can enable this feature flag by running in your console:

❯ echo "Feature.enable(:oncall_schedules_mvc)" | rails c

To check Enable the feature flag:

• Navigate to Operations => On-call Schedules or direct link is your_project/-/oncall_schedules e.g. http://127.0.0.1:3000/gitlab-org/gitlab-shell/-/oncall_schedules
• Click Add schedule in the empty state
• Add a base schedule with a name, description, and timezone
• You will see an empty grid with no rotations
• Click add a rotation on the top right of the grid
• Add a rotation with a handful of members that have access to the current project and pick a start date/time
• You should not be able to add a rotation without a title, valid assignees, and a start date/time

Screenshots (strongly suggested)

Validation 1	Validation 2

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 #322137