Forbid locale updates for scim-managed users.

[fisx]

https://wearezeta.atlassian.net/browse/WPB-9488 (see also: #4072)

Checklist

• [ ] Add a new entry in an appropriate subdirectory of changelog.d
• [ ] Read and follow the PR guidelines

[fisx]

this is not a security issue, and it will be fixed in https://github.com/wireapp/wire-server/pull/4046