Consider posting feature flag messages to Support on global rollout

Context

Prompted by production#5904 (closed) where support was not aware of the relevant feature flag being turned on

Requirements

Support would only want feature flag changes posted to the #support_gitlab-com channel for global production rollouts.

Criteria for pushing chatops/feature flag notifications:

• Environment: gprd
• Global: ON

Support also requires the following information which may not be the easier to do:

• Feature flag rollout issue link
• Description of possible impact to users

Note: Currently, it's a task item on the feature flag rollout template, so engineers are expected to post manually in order to include the required information.