feat: add service account to backup cronjob

[mmorejon]

Description

The service account is needed during the backup process to allow automatic authentication and authorization in public clouds like AWS, GCP or Azure. e.g.: to use workload identity in GKE. This fix avoids mounting secrets with credentials to access to public clouds.

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

How has this been tested?

Deployed in a local environment.

$ helm template demo \
  --dependency-update \
  --namespace default \
  ./charts/posthog/          

Checklist

• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works