fix(API username change): Issue from the pentest report

[samoehlert]

Remove the ability to change your own username from the API. This was pointed out in the pentest we had done :

The Users API found at ~/api/v1/users/<USER_NAME>/ allows for a user to change their username to one that doesn't exist. This doesn't have any major implications, however, it is unintended functionality that should be removed. As an example, if a user were to change their username to “Admïn,” a person could be fooled into thinking it was another user doing the changes.

[github-actions]

File	Coverage
All files	81%	✅
config/consumers.py	59%	✅
config/urls.py	69%	✅
config/settings/base.py	70%	✅
config/settings/local.py	73%	✅
scram/route_manager/admin.py	85%	✅
scram/route_manager/models.py	70%	✅
scram/route_manager/views.py	81%	✅
scram/route_manager/api/serializers.py	73%	✅
scram/route_manager/api/views.py	83%	✅
scram/templates/403.html	91%	✅
scram/templates/404.html	91%	✅
scram/templates/base.html	99%	✅

Minimum allowed coverage is 50%

Generated by 🐒 cobertura-action against 329a9ef

[crankynetman]

This looks perfect, thanks for fixing. I still don't know what's up with that dang coverage thing, but... Well, I'm fine ignoring it.