Enable prohibition of hex branch names

Changelog: added

config/feature_flags/development/prohibit_hexadecimal_branch_names.yml

----
-name: prohibit_hexadecimal_branch_names
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/commit/28439ca4b1dd14f22a5a6ad14530f6bf1046f8bc
-rollout_issue_url: 
-milestone: '12.10'
-type: development
-group: group::source code
-default_enabled: true

lib/gitlab/checks/branch_check.rb

@@ -41,7 +41,6 @@ module Gitlab
 
       def prohibited_branch_checks
         return if deletion?
-        return unless Feature.enabled?(:prohibit_hexadecimal_branch_names, project, default_enabled: true)
 
         if branch_name =~ /\A\h{40}\z/
           raise GitAccess::ForbiddenError, ERROR_MESSAGES[:prohibited_hex_branch_name]

spec/lib/gitlab/checks/branch_check_spec.rb

@@ -40,15 +40,6 @@ RSpec.describe Gitlab::Checks::BranchCheck do
           expect { subject.validate! }.not_to raise_error
         end
       end
-
-      context "the feature flag is disabled" do
-        it "doesn't prohibit a 40-character hexadecimal branch name" do
-          stub_feature_flags(prohibit_hexadecimal_branch_names: false)
-          allow(subject).to receive(:branch_name).and_return("267208abfe40e546f5e847444276f7d43a39503e")
-
-          expect { subject.validate! }.not_to raise_error
-        end
-      end
     end
 
     context 'protected branches check' do