Post note to monthly issue when post-deploy migration pipeline starts

What does this MR do and why?

Describe in detail what your merge request does and why.

Post a note to the monthly release issue when the post-deployment migration pipeline starts.

• The note will be posted by the post_deploy_migrations:prepare job.
• It will contain:
  • The name of the RM who triggered the pipeline.
  • A link to the pipeline.
• 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.

Example comment: gitlab-org/release/tasks#3629 (comment 925823276).

gitlab-com/gl-infra/delivery#2346 (closed)

Author Check-list

• [-] Has documentation been updated?