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

spec/lib/feed_manager_spec.rb

@@ -108,6 +108,7 @@ RSpec.describe FeedManager do
 
       it 'returns false for status by followee mentioning another account' do
         bob.follow!(alice)
+        jeff.follow!(alice)
         status = PostStatusService.new.call(alice, text: 'Hey @jeff')
         expect(FeedManager.instance.filter?(:home, status, bob.id)).to be false
       end