feat(migrations): Add new index to IncidentTrigger model

[isabellaenriquez]

This PR adds a new index to IncidentTrigger to make querying for the metric alert rate limiting feature more efficient #58032.

Query in question

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/0578_add_index_on_incident_trigger_alert_ratelimit.py ()

--
-- Alter index_together for incidenttrigger (1 constraint(s))
--
CREATE INDEX CONCURRENTLY "sentry_incidenttrigger_alert_rule_trigger_id_in_e98fd1e3_idx" ON "sentry_incidenttrigger" ("alert_rule_trigger_id", "incident_id");

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/0579_index_incident_trigger.py ()

--
-- Alter index_together for incidenttrigger (1 constraint(s))
--
CREATE INDEX CONCURRENTLY "sentry_incidenttrigger_alert_rule_trigger_id_in_e98fd1e3_idx" ON "sentry_incidenttrigger" ("alert_rule_trigger_id", "incident_id");