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

[Arsalanulhaq]

Summary

This pull request updates the group selection logic in the NewUserDialog.vue component to prevent users from being assigned as sub-admins of the admin group. The main change is the introduction of a new computed property to filter out the admin group from the selectable options for sub-admin assignment.

Group selection improvements:

• The :options prop for the sub-admin group selection now uses the new availableSubAdminGroups computed property, ensuring the admin group is excluded from the list of assignable sub-admin groups.
• Added the availableSubAdminGroups computed property, which filters out the admin group from availableGroups.

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)