Replace fa-times with GitLab SVG close icon - Dropdowns

What does this MR do?

This is one part of three MR's to replace all occurrences of the fa-times FontAwesome icon with the GitLab UI close icon SVG.

• !40585 (merged) (this MR)
• !40586 (merged)
• !40587 (merged)

This MR specifically replaces the use of the icon in dropdown related contexts.

Screenshots

Location	With FA icon	With SVG icon
[HOST]/[GROUP]/[PROJECT]/-/issues/[ID] _Edit --> Description templates dropdown_
[HOST]/[GROUP]/[PROJECT]/-/clusters/new?provider=gcp _Select project (Removed as it isn't used)_
[HOST]/[GROUP]/[PROJECT]/-/boards _Edit board --> Labels --> Create label_
[HOST]/[GROUP]/[PROJECT]/-/boards _Edit board --> Labels --> Create label_
[HOST]/[GROUP]/[PROJECT]/-/issues/[ID] _Sidebar --> Labels_
[HOST]/[GROUP]/[PROJECT]/-/issues/[ID] _Sidebar --> Labels --> Create project label_
[HOST]/[GROUP]/[PROJECT]/-/issues/[ID] _Sidebar --> Assignees_
[HOST]/[GROUP]/[PROJECT]/-/issues/[ID] _Sidebar --> Epics_
[HOST]/[GROUP]/[PROJECT]/-/issues/[ID] _Sidebar --> Epics upgrade plan link (requires the promotion to be enabled)
[HOST]/[GROUP]/[PROJECT]/-/issues/new _Description templates link (requires the promotion to be enabled)

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

You can either use the paths noted in the screenshots above or review the screenshots themselves. Some of these are very tricky to get to and either require you to have your instance set up a specific way or editing the code to force their display 🙈

• 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 #224534 (closed)