Zoekt index assignments should factor in index claimed storage

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Problem

During index node assignments, we look at the node's unused storage bytes. This leads us to assigning reservations to indices that have more reservation in total than the node can support.

See: https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/app/services/search/zoekt/scheduling_service.rb#L229

Proposal

During index node assignments, we should look at the nodes unclaimed storage bytes before allow an index to make a reservation.