Update SettleMint Helm registry reference in docs (#803)

## Summary
- update the Helm chart OCI reference to
harbor.settlemint.com/settlemint/settlemint
- bump the documented chart version to v7.31.11

## Testing
- no tests were run (not applicable)

------
https://chatgpt.com/codex/tasks/task_b_68ece6bccbac832bb911313276a31704
    
<!-- This is an auto-generated description by cubic. -->
---

## Summary by cubic
Update on-prem install docs to use the new Helm OCI registry and chart
version. This ensures users install from the correct source and avoid
failures.

All examples now use oci://harbor.settlemint.com/settlemint/settlemint
with --version v7.31.11.

<!-- End of auto-generated description by cubic. -->

Author: bl0up

content/docs/launching-the-platform/self-hosted-onprem/platform-installation.mdx

@@ -114,7 +114,7 @@ Enterprise customers receive dedicated registry credentials with access to enter
 View all available configuration options for enterprise deployment:
 
 ```bash
-helm show values oci://registry.settlemint.com/settlemint-platform/SettleMint --version 7.0.0
+helm show values oci://harbor.settlemint.com/settlemint/settlemint --version v7.31.11
 ```
 
 <Callout type="warning">
@@ -537,9 +537,9 @@ support:
 Install the platform:
 
 ```bash
-helm upgrade --install SettleMint oci://registry.settlemint.com/settlemint-platform/SettleMint \
+helm upgrade --install SettleMint oci://harbor.settlemint.com/settlemint/settlemint \
   --namespace SettleMint \
-  --version 7.0.0 \
+  --version v7.31.11 \
   --create-namespace \
   --values values.yaml
 ```
@@ -779,7 +779,7 @@ If you encounter issues during installation:
 1. Debug the installation:
 
 ```bash
-helm upgrade --install --debug --dry-run SettleMint oci://registry.settlemint.com/settlemint-platform/SettleMint \
+helm upgrade --install --debug --dry-run SettleMint oci://harbor.settlemint.com/settlemint/settlemint \
   --namespace SettleMint \
   --values values.yaml
 ```