Post a message on the release issue when post-deploy pipeline is executed

As part of &585 (closed).

In the same way, a message is posted whenever a gprd deployment starts, we should post a message when the post-deploy pipeline is executed.

The message should:

• State who started the pipeline
• Production checks status.

Implementation

This change is behind a feature flag called post_deploy_migrations_exec_start_note: https://ops.gitlab.net/gitlab-org/release/tools/-/feature_flags/220/edit.