User Popover: migrate BS utility classes to GitLab UI utility classes

What does this MR do?

See #227088 (closed)

As I was migrating the utility classes I noticed a CSS regression in the alignment of the location icon (see screenshots). This is also fixed in this MR.

Screenshots

Component	Before	After
Main User Popover
Requirements User Popover

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry

  • Any developer-facing change (e.g., refactoring, technical debt remediation, test suite changes) should not have a changelog entry. Example: “Reduce database records created during Cycle Analytics model spec.”

  • A fix for a regression introduced and then fixed in the same release (i.e., fixing a bug introduced during a monthly release candidate) should not have a 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