Move FF zoekt_reduce_parallelism to the zoekt admin setting

Background

The FF zoekt_reduce_parallelism controls the parallel indexing threads in the zoekt webserver. Currently, it is hardcoded to 1 which does not give flexibility to self-managed customers.

Proposal

Move the FF zoekt_reduce_parallelism to zoekt admin settings. https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/models/search/zoekt/settings.rb

The field can be number_field