refactor: set default replicas to 1 for secondary storage

[EuroLew]

Which problem does the PR fix?

There is an issue where if one ES pod goes down due to replicas = 0 it will block the main application, to stop secondary storage blocking like this and make Camudna more HA replicas should be 1 for unified indices. camunda/camunda#35080

This is a breaking change for the upgrade from 8.7->8.8, I want this to be in the helm chart for 8.8 upgrade is the way I'm doing it correct?

This is also paired with a documentation PR that tells the user about the need for more space and how to rollback. camunda/camunda-docs#6664. The rollback has been tested and works in the gke-4150-intg-8-8-gke-eske namespace

Checklist

Please make sure to follow our Contributing Guide.

Before opening the PR:

• In the repo's root dir, run make go.update-golden-only.
• There is no other open pull request for the same update/change.
• Tests for charts are added (if needed).
• In-repo documentation are updated (if needed).

After opening the PR:

• [] Did you sign our CLA (Contributor License Agreement)? It will show once you open the PR.
• Did all checks/tests pass in the PR?

[bkenez]

Looks good, just some wording/naming change suggestions 👍

[hisImminence]

@aabouzaid, pinging you for the final review as marked as breaking change!

[aabouzaid]

@EuroLew I've ported the change in the unified config for the Camunda Exporter, but number-of-replicas is still not supported in the Secondary Storage config.

Please confirm that having it for Camunda Exporter only is enough.