Improve Slack warning message when a rollback includes security or high severity fixes

Context

This is spun off from gitlab-org/release-tools!4410 (comment 2753960719).

In gitlab-org/release-tools!4410 (merged), we made an important improvement for the rollback check ChatOps command, so in the future the Slack message will also warn us if the rollback includes security or incident fixes.

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This issue suggests an improvement to the message:

  • Move the "⚠️ WARNING" block to the top of the message
  • Change the rollback availability in such case from " 🟢 Rollback available" to "⚠️ Rollback available with WARNING"

This improve visibility for the message.