Reschedule async temp index on finding report types

What does this MR do and why?

We have unintentionally removed a temporary index that was introduced for a background migration, before the background migration has executed.

The background migration has been paused on production. Once the temporary index is back in, it'll get resumed.

• Background migration introduced in 15.10: !113348 (merged)
• Temporary index introduced in 15.10: !113348 (merged)
• Temporary index removed in 15.11: !116230 (merged)

This MR is one of 3 that restores the temporary index for 15.11:

• !116540 (merged) turns the removal migration into a no-op
• !116522 (merged) re-schedules the temporary index to be built, if already removed
• !116529 (merged) synchronises the temporary index once built asynchronously

DB

Index creation:

CREATE INDEX CONCURRENTLY "tmp_idx_vulnerability_occurrences_on_id_where_report_type_7_99" ON "vulnerability_occurrences" ("id") WHERE report_type IN (7, 99)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #359963 (closed)