Schedule async temp index creation on finding report types

What does this MR do and why?

Schedules async creation of a temporary index for the background migration in !112556 (merged):

CREATE INDEX tmp_idx_vulnerability_occurrences_on_id_where_report_type_7_99 ON vulnerability_occurrences USING btree (id) WHERE (report_type = ANY (ARRAY[7, 99]));

Built in ~80 mins on postgres.ai

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)