Refactor MTE-5260 Use json schedule to determine rotation.

[clarmso]

After some discussion with Matt R, here are the changes in the security monitor notification tool:

• The rotation schedule is in the ios_sec_triage.json.
• Github Actions workflow will run every weekday.
• If today's date matches one of the duty-start-dates, emit a new message to the Slack channel.
• If there's no new assignee today, do not send a Slack message.
• The Slack message will ping the new assignee using @ [Slack Handle].