Support link regression in 115.1.0esr

During the rebase of alpha, I've found that Mozilla reworked the way they to assign URLs to support pages.

I think they re-route everything on the server side, and the new logic is just to append a path to a base URL.

We already have an issue about properly handling the various support URLs, but this is different because this results in a regression for the support button on the bottom-left corner of about:preferences.

Maybe the old code still works, but it risks of becoming a fight with upstream. Probably we should do something more ad-hoc in toolkit/content/widgets/moz-support-link/moz-support-link.mjs (check #setHref).

In addition, Firefox also appends a utm-content as a sort of telemetry to understand where the users click to get to that support page. Maybe we'll want to clean the generated URL in any case. This might benefit also Base Browser/Mullvad Browser.