fix: Fix Twitter fetching

Twitter now returns a 429 HTTP error code when fetching
with Googlebot-compatible user agent. I don’t know if it's temporary,
but it seems to work with a simple `compatible; bot` user agent, so
let's try it.

src/services/LinkFetcher.php

@@ -161,7 +161,7 @@ public function fetchUrl($url)
                 // If we fetch Twitter, we need to alter our user agent to get
                 // server-side rendered content.
                 // @see https://stackoverflow.com/a/64332370
-                $options['user_agent'] = $this->http->user_agent . ' (compatible; Googlebot/2.1)';
+                $options['user_agent'] = $this->http->user_agent . ' (compatible; bot)';
             }
 
             if ($selected_ip) {