Wiki repositories use default branch when creating the repo

In this commit, we allow wiki repositories to use the instance default
branch when creating the repo. If not default branch is set,
we will default to the code default ref which, at this point, is
`main`.

Changelog: added

app/models/wiki.rb

@@ -249,8 +249,6 @@ class Wiki
 
   override :default_branch
   def default_branch
-    return 'master' if Feature.disabled?(:wiki_uses_default_branch, user, default_enabled: :yaml)
-
     super || wiki.class.default_ref(container)
   end
 

config/feature_flags/development/wiki_uses_default_branch.yml

----
-name: wiki_uses_default_branch
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/64891
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/334754
-milestone: '14.1'
-type: development
-group: group::editor
-default_enabled: false

spec/support/shared_examples/models/wiki_shared_examples.rb

@@ -545,22 +545,12 @@ RSpec.shared_examples 'wiki model' do
       allow(Gitlab::DefaultBranch).to receive(:value).and_return('main')
     end
 
-    shared_examples 'feature flag wiki_uses_default_branch is disabled' do
-      it 'returns "master"' do
-        stub_feature_flags(wiki_uses_default_branch: false)
-
-        expect(subject).to eq 'master'
-      end
-    end
-
     context 'when repository is not created' do
       let(:wiki_container) { wiki_container_without_repo }
 
       it 'returns the instance default branch' do
         expect(subject).to eq 'main'
       end
-
-      it_behaves_like 'feature flag wiki_uses_default_branch is disabled'
     end
 
     context 'when repository is empty' do
@@ -573,8 +563,6 @@ RSpec.shared_examples 'wiki model' do
       it 'returns the instance default branch' do
         expect(subject).to eq 'main'
       end
-
-      it_behaves_like 'feature flag wiki_uses_default_branch is disabled'
     end
 
     context 'when repository is not empty' do