MM-32013: Do not allow password change in /auth endpoint (#16770)

We were allowing new password to be reset without asking
for the old password. This was not advertised but was possible nevertheless
through the API.

Since there is already a separate API to change password, we choose to remove
this functionality.

https://mattermost.atlassian.net/browse/MM-32013

```release-note
The /api/v4/users/me/auth API endpoint cannot be used to change password anymore.
This was a hidden feature that was not documented, but was nevertheless possible.
We are just removing the hidden feature.
```

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

i18n/en.json

@@ -3486,6 +3486,10 @@
     "id": "api.user.update_user.accepted_guest_domain.app_error",
     "translation": "The email you provided does not belong to an accepted domain for guest accounts. Please contact your administrator or sign up with a different email."
   },
+  {
+    "id": "api.user.update_user_auth.invalid_request",
+    "translation": "Request is missing either AuthData or AuthService parameter."
+  },
   {
     "id": "api.user.update_user_roles.license.app_error",
     "translation": "Custom Permission Schemes not supported by current license"
@@ -5690,10 +5694,6 @@
     "id": "app.user.update_failed_pwd_attempts.app_error",
     "translation": "Unable to update the failed_attempts."
   },
-  {
-    "id": "app.user.update_password.app_error",
-    "translation": "Unable to update the user password."
-  },
   {
     "id": "app.user.update_thread_follow_for_user.app_error",
     "translation": "Unable to update following state for thread"