Enable sidekiq gitaly-throttled db load-balancing

REVIEW ONLY; DON'T MERGE

What does this MR do?

Sets environment variable ENABLE_LOAD_BALANCING_FOR_SIDEKIQ to 'true' for gprd in order to enable load balancing for the gitaly-throttled sidekiq shard in production.

For more details see: gitlab-org/gitlab#327107 (closed)

This is being done in order to enable the use of a load-balanced database connection pool for asynchronous sidekiq requests so that request load is spread out across all database cluster read-replica members.

Author Check-list

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Reviewer Check-list

• Reviewed the diff jobs to confirm changes are as expected
• No changes shown in the diffs not associated with this MR - This may require a rebase or further investigation