Link change management for feature flag rollout in issue template

This adds a link to the change management documentation for feature flags.

Not all feature flags need an accompanying change issue, but we make sure we don't forget to check if it matches any of the criteria mentioned on https://about.gitlab.com/handbook/engineering/infrastructure/change-management/#feature-flags-and-the-change-management-process. If it does, a change management issue ensures that the infrastructure team is well aware of what is happening, allowing them to act fast if something goes south.

Brought up when rolling out #326095 (comment 561731598)