Synchronize 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

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)