Add API endpoint to change a user's language

[mschoettle]

The app allows the user to change the language in the app which also gets stored in the User model. However, currently, it only gets stored in the legacy DB causing deviation errors.

There is no API endpoint that can be called to change the user's language.

The options are:

1. Provide a dedicated endpoint to change the language, e.g., /caregivers/profile/change-language/
2. Provide an update view along with CaregiverProfileView that allows one for now to only change the language. It could then be extended with other fields in the future.