Resolve "Admin Settings: Cannot read property 'addEventListener' of null"

What does this MR do?

This MR makes sure that initUserInternalRegexPlaceholder is loaded only when needed (i.e., on the admin settings General page) instead of loading it on all admin subpages.

By this, it fixes the issue of attaching an event handler to a non-existent DOM element but also avoids loading of unnecessary JS code on certain admin pages.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/56622

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #56622 (closed)