Consider adding index for OncallRotations on starts_at/ends_at based on table's usage

Context

Query analysis for !53675 (merged) indicated an index would not be immensely useful for the combination of starts_at and ends_at fields on the table incident_management_oncall_rotations.

However, usage of these features may alter that. If many users create rotations with end dates, it could be possible that an index will increase in usefulness.

Task

Perform a secondary performance analysis for the query used to persist shifts. This is currently IncidentManagement::OncallRotations.in_progress.pluck(:id), but perhaps this has changed.

Determine whether any indexes or other improvements can be made to this query.