utils/source-discoverer: Fix HTTP errors interpretation

There can be many reasons why a request is not allowed or authorized.
403 and 405 HTTP error codes do not prompt the visitor for credentials.
Only 401 and 407 responses are credentials prompts.

Only raise a username and password request when facing a 401 or 407.

Fixes: https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/645
Fixes: https://gitlab.gnome.org/GNOME/gnome-calendar/-/issues/552

src/utils/gcal-source-discoverer.c

@@ -199,9 +199,9 @@ is_authentication_error (gint code)
 {
   switch (code)
     {
+    /* Only HTTP 401 and 407 errors are actual prompts for credentials. 403, 405, etc. are not. */
     case SOUP_STATUS_UNAUTHORIZED:
-    case SOUP_STATUS_FORBIDDEN:
-    case SOUP_STATUS_METHOD_NOT_ALLOWED:
+    case SOUP_STATUS_PROXY_UNAUTHORIZED:
       return TRUE;
     }