Make `NotifyFeatureFlagMrDeployment` react to `default_enabled` state

What does this MR do and why?

This improves the current NotifyFeatureFlagMrDeployment processor to post different messages based on the default_enabled state of the feature flag:

• default_enabled was set to true: the message will incentivize the author to delete the feature flag record in the latest deployed environment
• default_enabled was set to false: the message will incentivize the author to enable the feature flag in the latest deployed environment

Both messages include the actual chatops command to run.

This will also post the messages in the same discussion, so that the timeline of feature flag state change is grouped in the same thread.

Action items

• If adding environment variables for reactive processors, update config/triage-web.yaml and .gitlab/ci/triage-web.yml
• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review