chore(alerts): Add index on date_added, soft remove metric alert colu…

[ceorourke]

Follow up to #115578 to start removal of the metric alert columns and add an index on date_added to prep for adding a cleanup job on the table for rows over 90 days old.

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/notifications/migrations/0011_pending_delete_metric_alert_cols_notifmsg.py

for 0011_pending_delete_metric_alert_cols_notifmsg in notifications

--
-- Moved notificationmessage.incident field to pending deletion state
--
-- (no-op)
--
-- Moved notificationmessage.trigger_action field to pending deletion state
--
-- (no-op)
--
-- Create index sentry_noti_date_ad_ac82c5_idx on field(s) date_added of model notificationmessage
--
CREATE INDEX CONCURRENTLY "sentry_noti_date_ad_ac82c5_idx" ON "sentry_notificationmessage" ("date_added");

[wedamija]

lgtm, this will need to be post deploy though since the index will take a while.

The soft removals are a no-op so it'll be fine to have them run as part of the post deploy