Error 405 for unsupported keyset pagination calls

This is in line with what we document:
https://docs.gitlab.com/ee/api/README.html#status-codes

lib/api/helpers/pagination.rb

@@ -9,7 +9,7 @@ module API
         request_context = Gitlab::Pagination::Keyset::RequestContext.new(self)
 
         unless Gitlab::Pagination::Keyset.available?(request_context, relation)
-          return error!('Keyset pagination is not yet available for this type of request', 501)
+          return error!('Keyset pagination is not yet available for this type of request', 405)
         end
 
         Gitlab::Pagination::Keyset.paginate(request_context, relation)

spec/lib/api/helpers/pagination_spec.rb

@@ -47,7 +47,7 @@ describe API::Helpers::Pagination do
       it 'renders a 501 error if keyset pagination isnt available yet' do
         expect(Gitlab::Pagination::Keyset).to receive(:available?).with(request_context, relation).and_return(false)
         expect(Gitlab::Pagination::Keyset).not_to receive(:paginate)
-        expect(subject).to receive(:error!).with(/not yet available/, 501)
+        expect(subject).to receive(:error!).with(/not yet available/, 405)
 
         subject.paginate(relation)
       end

spec/requests/api/projects_spec.rb

@@ -632,7 +632,7 @@ describe API::Projects do
         it 'responds with 501 if order_by is different from id' do
           get api('/projects', current_user), params: params.merge(order_by: :created_at)
 
-          expect(response).to have_gitlab_http_status(501)
+          expect(response).to have_gitlab_http_status(405)
         end
       end