Refactor group syncing on user edit API endpoint

[snipe]

This tightens up the code around syncing user group membership when using the API and adds a bunch of tests for conditions we weren't testing before.

[what-the-diff]

PR Summary

• Improved User Permission Handling
  The file app/Http/Controllers/Api/UsersController.php is now more robust with added clarifications for user permissions processing. We've also enhanced the conditions for user group assignments to account for superusers.
• User Interface Text Changes
  In resources/views/groups/view.blade.php, we've updated the text to offer a clearer user interface.
• Added Test Cases for User Management
  In tests/Feature/Api/Users/UpdateUserApiTest.php, we've added new test scenarios to comprehensively check non-superuser cases as well as superuser group assignments. This will make sure our user management functionality is more accurate and reliable.

[marcusmoore]

Looks good! There's a small variable typo but other than that

[uberbrady]

This looks great! Nice cleanups here.