Update service desk project to use GlLoadingIcon instead of font awesome spinner

What does this MR do?

This merge is for issue #198466 (closed)

Its the minimal viable change to replace the font awesome spinner <i class="fa fa-spinner fa-spin" aria-hidden="true"></i> inside the ee/app/assets/javascripts/projects/settings_service_desk/ app with the correct GlLoadingIcon.

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers