Update feature flag template to make staging-canary explicit

Part of https://gitlab.com/gitlab-com/gl-infra/delivery/-/issues/2280

With the change of the deployment pipeline for GitLab.com at

https://gitlab.com/groups/gitlab-com/gl-infra/-/epics/608

`staging-canary` is now the first environment deploy to, and this
the best place to test new features and feature flags.

This change updates the feature flag issue template to call this out
explicitly so developers now that is where they should be testing
their feature flags before production.

.gitlab/issue_templates/Feature Flag Roll Out.md

@@ -65,6 +65,9 @@ _Consider adding links to check for Sentry errors, Production logs for 5xx, 302s
     - [ ] `/chatops run feature set <feature-flag-name> true --dev`
     - [ ] `/chatops run feature set <feature-flag-name> true --staging`
 - [ ] Verify that the feature works as expected. Posting the QA result in this issue is preferable.
+      The best environment to validate the feature in is [staging-canary](https://about.gitlab.com/handbook/engineering/infrastructure/environments/#staging-canary)
+      as this is the first environment deployed to. Note you will need to make sure you are configured to use canary as outlined [here](https://about.gitlab.com/handbook/engineering/infrastructure/environments/canary-stage/)
+      when accessing the staging environment in order to make sure you are testing appropriately.
 
 ### Specific rollout on production