Migrate `GlDeprecatedSkeletonLoading` to `GlSkeletonLoader`

📖 What does this MR do?

Related to #244694 (closed)
Part of &4301 (closed)

Migrates GlDeprecatedSkeletonLoading to GlSkeletonLoader in the user popover.

💻 Local testing

1. Navigate to a MR or issue
2. Open the developer tools (⌘ + option + I in Chrome)
3. In the Network tab of the developer tools there is a Throttling dropdown. Choose Slow 3G
4. Hover over a user's name

You can also see the loading state by changing app/assets/javascripts/vue_shared/components/user_popover/user_popover.vue#L69 to true

📹 Videos

With network throttled to 3G so it shows GlSkeltonLoader longer.

Before	After
Screen_Recording_2021-04-12_at_11.33.16_AM	Screen_Recording_2021-04-12_at_11.25.54_AM

🚦 Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• [-] 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