Add loading indicator to "Update username" button in account settings

What does this MR do?

This MR adds a loading spinner to the "Update username" button in the account settings (/-/profile/account) in the "Change username" section.

Related to #21759 (closed)

Screenshots (strongly suggested)

VD: the video shows the user clicking on a button titled "Update username". This opens a modal, where the users click on another button with the same title. The modal is closed and the first button now shows a loading indicator.

Before	After

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