Add ability to configure number of replicas in Gitaly's statefulset

What does this MR do?

By default, Gitaly's statefulset replicas is set to the length of gitaly.internal.names. We want to be able to configure this number.

statefulset:
  replicas: 2

Providing this in the values.yaml of Gitaly will override the number of replicas.

Testing from root of repo:

❯ helm template . --debug --values=values.yaml --values=charts/gitlab/charts/gitaly/values.yaml --show-only charts/gitlab/charts/gitaly/templates/statefulset.yml --set certmanager-issuer.email=em@example.com > y.yml

Related issues

Relates to gitlab-com/gl-infra/k8s-workloads/gitlab-com!3841 (merged)

Enables us to set the Gitaly to 1 replica per STS

Author checklist

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Required

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• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated, and test plan for scenarios not covered by automated tests.
• Equivalent MR/issue for omnibus-gitlab opened.

Reviewers checklist

• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab Chart.