core/exec-credential: downgrade warning about missing cred to debug

Follow-up for 6b34871.

The general idea is that the list of credentials to load can and will specify
credentials which actually aren't provided, so a warning is too much. Let's
downgrade this to "info". If it turns out to be too noisy, we can downgrade
further in the future.

Author: keszybz

src/core/exec-credential.c

@@ -504,7 +504,7 @@ static int load_credential(
                  *
                  * Also, if the source file doesn't exist, but a fallback is set via SetCredentials=
                  * we are fine, too. */
-                log_full_errno(hashmap_contains(context->set_credentials, id) ? LOG_DEBUG : LOG_WARNING,
+                log_full_errno(hashmap_contains(context->set_credentials, id) ? LOG_DEBUG : LOG_INFO,
                                r, "Couldn't read inherited credential '%s', skipping: %m", path);
                 return 0;
         }