Draft: Lower Geo replication concurrency default settings

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What does this MR do and why?

Caution

This MR was originally created at !184473 (merged) via gitlab-community but we were getting strange CI problems there. Thus an MR with the exact same commits was raised via gitlab-org instead.

Shifted focus again to the original MR !184473 (merged) to investigate possible CI performance leaks and concluding the activities.

This MR lowers the Geo-related concurrency default settings as outlined in #24081 (closed). By lowering the concurrency default values, smaller environments will profit from a more balanced resource utilization. Larger deployments most likely will have to further tweak those options to avoid replication lag.

As suggested in #24081 (closed), the docs changes are handled in a separate MR (see !184510 (merged)).

Notes:

• The issue also mentions files_max_capacity but its default value is already set to 10

TODO:

• Migration spec tests?
• Tuning docs?
• Add columns_changing_default?
• Changelog?

🔧 with at Adfinis

References

• #24081 (closed)
• !184510 (merged)
• (#393570)

Screenshots or screen recordings

n/a

How to set up and validate locally

At least run the newly defined spec tests:

bundle exec rspec spec/migrations/db/migrate/20250312124044_change_geo_performance_default_settings_spec.rb

MR acceptance checklist

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