Highlight the reason why a rollback is not available

What does this MR do?

Highlight the reason why a rollback is not available in the rollback check command

If there's a single reason, the reason is included in the header. If there is more than one reason (deployment in progress and post-deploy migrations included) a new section is included.

As part of gitlab-com/gl-infra/delivery#2080

Examples

With a deployment in progress	With post-deploy migrations	With a deployment in progress and post-deploy migrations
Slack Blocks	Slack blocks	Slack blocks

• Note that for the last two examples the post-migrations block is missing, this is intentional as the images were generated using the spec data.

Author Check-list

• Has documentation been updated?