Allow setting load_balancing.discover.max_replica_pools

What does this MR do?

This was added in gitlab-org/gitlab!101994 (merged) and it is already documented in the GitLab docs at https://docs.gitlab.com/ee/administration/postgresql/database_load_balancing.html#service-discovery . This change allows us to pass this through via the Helm chart.

Related issues

1. gitlab-org/gitlab#368625 (closed)
2. gitlab-com/gl-infra/production#7993 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• Merge Request Title and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• When ready for review, MR is labeled "~workflow::ready for review" per the Distribution MR workflow

Expected (please provide an explanation if not completing)

See !2841 (comment 1161780160) for questions/explanations for the below items.

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for omnibus-gitlab opened
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.