[FIX] webservice: WARNING message in logs

The use of a compute method on `oauth2_flow` when this field is touched by
the server environment mixin causes it to be defined twice as computed,
with differents settings, and this ultimately causes a warning message
in the logs:

```
WARNING odoo odoo.modules.registry: webservice.backend: inconsistent 'compute_sudo' for computed fields: protocol, url, auth_type, username, password, api_key, api_key_header, oauth2_flow, oauth2_clientid, oauth2_client_secret, oauth2_token_url, oauth2_authorization_url, oauth2_audience, oauth2_scope, content_type
```

We fix this by using an old fashioned onchange declaration on the
`auth_type` field.

webservice/models/webservice_backend.py

@@ -39,7 +39,6 @@ class WebserviceBackend(models.Model):
         ],
         readonly=False,
         store=True,
-        compute="_compute_oauth2_flow",
     )
     oauth2_clientid = fields.Char(string="Client ID", auth_type="oauth2")
     oauth2_client_secret = fields.Char(string="Client Secret", auth_type="oauth2")
@@ -121,8 +120,10 @@ def _get_adapter_protocol(self):
             protocol += f"+{self.auth_type}-{self.oauth2_flow}"
         return protocol
 
-    @api.depends("auth_type")
-    def _compute_oauth2_flow(self):
+    @api.onchange("auth_type")
+    def _onchange_oauth2_auth_type(self):
+        # reset the auth2_flow when auth_type is not oaut2
+        # using a compute method interfers with the server environment mixin
         for rec in self:
             if rec.auth_type != "oauth2":
                 rec.oauth2_flow = False