Increase ProcessBookkeepingService batch to 10_000

Since we've learnt through monitoring that batches with 1000 jobs are
not taking more than 5.5s on average and the median is around 4s we
should be save to increase this ten times and still process batches in
the desired time window.

This started from a conversation at
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/28511#note_315475648
where initially we wanted this to be configurable but really it turns
out this number should be approximately how quickly a single core can
process this many jobs being marshalled and sent to Elasticsearch and is
likely not going to be something that benefits much from configuration.
But we'll want to increase it now to 10k which still seems like a safe
number and will give us a lot of scaling headroom before we have to
figure out how to parallelize this work.

ee/app/services/elastic/process_bookkeeping_service.rb

@@ -4,7 +4,7 @@ module Elastic
   class ProcessBookkeepingService
     REDIS_SET_KEY = 'elastic:incremental:updates:0:zset'
     REDIS_SCORE_KEY = 'elastic:incremental:updates:0:score'
-    LIMIT = 1000
+    LIMIT = 10_000
 
     class << self
       # Add some records to the processing queue. Items must be serializable to

ee/changelogs/unreleased/increase-bulk-indexer-batch-sizes.yml

+---
+title: Increase ProcessBookkeepingService batch to 10_000
+merge_request: 30817
+author:
+type: changed