added `dropdown-extended-height` class to labels dropdown-menu

What does this MR do?

This MR does add the dropdown-extended-height class to the labels dropdown, which increases the max-height for it from the original 312px to 400px according to this proposal: Increase assign labels dropdown to 400px.

@dennis please feel free to comment/criticize since this is my first MR.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and testing

There is no actual risk to the UI since the change only incorporates a increased max-height (no new class/property)

• 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