fix: remove revisionHistoryLimit from statefulset-mail.yaml

[scthi]

remove not allowed revisionHistoryLimit property from statefulset-mail.yaml

causing Helm error:

Helm upgrade failed: cannot patch "kratos-courier" with kind StatefulSet: StatefulSet.apps "kratos-courier" is invalid: spec: Forbidden: updates to statefulset spec for fields other than 'replicas', 'ordinals', 'template', 'updateStrategy', 'persistentVolumeClaimRetentionPolicy' and 'minReadySeconds' are forbidden

Checklist

• I have read the contributing guidelines and signed the CLA.
• I have referenced an issue containing the design document if my change introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security vulnerability.
  If this pull request addresses a security vulnerability,
  I confirm that I got approval (please contact security@ory.sh) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works.
• I have added the necessary documentation within the code base (if appropriate).

Further comments

[Demonsthere]

This field is not disallowed, but statefulsets are immutable, meaning you cannot modify most fields during an upgrade.
If you cannot recreate your statefulset, it is enough to set the revisionHistoryLimit to the default 10 :)

[Demonsthere]

Closing as Won't Fix