DEV: update dependencies and add notes about exceptions

Previously it was unclear why certain gems are being held back cause Gemfile
had no comment explaining it.

I tried to add some explanation from memory and remove some exceptions that
seemed to be superfluous.

This upgrades shoulda to latest, it appears to work once a couple of assertions
are removed

Also update http accept language used to auto detect language from http header
this is tested

Zeitwerk small update seems fine

spec/models/post_reply_spec.rb

@@ -7,9 +7,6 @@ describe PostReply do
   fab!(:post) { Fabricate(:post, topic: topic) }
   fab!(:other_post) { Fabricate(:post, topic: topic) }
 
-  it { is_expected.to belong_to :post }
-  it { is_expected.to belong_to :reply }
-
   context "validation" do
     it "should ensure that the posts belong in the same topic" do
       expect(PostReply.new(post: post, reply: other_post)).to be_valid