Improvements for the security release Slack notification

When processing security merge requests through the /chatops run release merge --security command, a notification is sent to Slack informing what security issues are pending and which ones were processed:

Although informative, the command output is not super helpful:

• It's not accurate - The security issues reported - 20 on the image above - don't match the ones listed on Pending and Processed sections.
• It implies additional work - Aside from triggering the command, the release manager needs to know which merge requests were already merged. Having this information handy helps release managers to continue with the next steps of the security release https://gitlab.com/gitlab-org/release/tasks/-/issues/3365#note_820165496

Proposal

Update the output of the /chatops run release merge --security to return a comment indicating the security issues' status. Example:

Processed

Security issues

issue	master merged?	backports merged?	Notes
https://gitlab.com/gitlab-org/security/gitlab/-/issues/123	✅
https://gitlab.com/gitlab-org/security/gitlab/-/issues/345	✅	✅
https://gitlab.com/gitlab-org/security/gitlab/-/issues/679			Not ready

Details

1. The comment should be posted in the Security Release task issue (on release/tasks)
2. First execution of the release merge command will post a comment and build the table.
3. Upcoming executions will update the same comment.
4. Same comment should be used for both version of the security command (with --default-branch and without it)