Remove dependency on goldfinger gem (#14919)

There are edge cases where requests to certain hosts timeout when
using the vanilla HTTP.rb gem, which the goldfinger gem uses. Now
that we no longer need to support OStatus servers, webfinger logic
is so simple that there is no point encapsulating it in a gem, so
we can just use our own Request class. With that, we benefit from
more robust timeout code and IPv4/IPv6 resolution.

Fix #14091

app/services/process_mentions_service.rb

@@ -29,7 +29,7 @@ class ProcessMentionsService < BaseService
       if mention_undeliverable?(mentioned_account)
         begin
           mentioned_account = resolve_account_service.call(Regexp.last_match(1))
-        rescue Goldfinger::Error, HTTP::Error, OpenSSL::SSL::SSLError, Mastodon::UnexpectedResponseError
+        rescue Webfinger::Error, HTTP::Error, OpenSSL::SSL::SSLError, Mastodon::UnexpectedResponseError
           mentioned_account = nil
         end
       end