Replace fa icons with svg icons in Board modal

What does this MR do?

This MR replaces font awesome icons in the boards modal with the SVG sprite.

Part of #224527 (closed), #224509 (closed) and #224534 (closed)

It replaces the following icons:

Fontawesome	SVG sprite
fa-chevron-down	chevron-down
fa-search	search
fa-times	close

Screenshots

Component	Before	After
Milestone Dropdown (fa-chevron, fa-search)
Milestone Dropdown (fa-times)
Labels Dropdown (fa-chevron, fa-search)
Labels Dropdown (fa-times)
Assignee Dropdown (fa-chevron, fa-search)
Assignee Dropdown (fa-times)

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