Fix hard-coded IDs in failing metrics_dashboard_spec.rb

`spec/requests/projects/metrics_dashboard_spec.rb` was failing because
it was expecting environment ID 99 not to exist, but if previous specs
incremented the sequence then it's entirely possible for this spec to
create an environment with that ID. We now use the max value to test
this.

spec/requests/projects/metrics_dashboard_spec.rb

@@ -70,7 +70,7 @@ RSpec.describe 'Projects::MetricsDashboardController' do
 
     context 'when query param environment does not exist' do
       it 'responds with 404' do
-        send_request(environment: 99)
+        send_request(environment: non_existing_record_id)
         expect(response).to have_gitlab_http_status(:not_found)
       end
     end
@@ -105,7 +105,7 @@ RSpec.describe 'Projects::MetricsDashboardController' do
 
     context 'when query param environment does not exist' do
       it 'responds with 404' do
-        send_request(dashboard_path: dashboard_path, environment: 99)
+        send_request(dashboard_path: dashboard_path, environment: non_existing_record_id)
         expect(response).to have_gitlab_http_status(:not_found)
       end
     end