fix(NewUserDialog): update group selection to prevent admin group from being assigned as subadmin

[Arsalanulhaq]

Summary

This pull request makes a small but important adjustment to the new user dialog in the settings app. The main change is to exclude the admin group from the list of groups that can be assigned as sub-admin groups when creating a new user.

Group selection logic updates:

• Added a new computed property availableSubAdminGroups in NewUserDialog.vue that filters out the admin group from the available groups.
• Updated the group selection dropdown to use availableSubAdminGroups instead of availableGroups, preventing the admin group from being assigned as a sub-admin group.

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)