Test cluster installation on Rocky Linux 9 (escluster)

[cbaxley]

Part of: #737 (Eng Spec: Add clustering support for Elasticsearch)

Description
As a maintainer, I want to verify cluster installation works on Rocky Linux 9 so that users on this OS have a supported deployment path.

Acceptance criteria

• Master node installation completes successfully on Rocky Linux 9
• Cluster node installation completes successfully on Rocky Linux 9
• All nodes join cluster and report healthy
• Certificates are properly distributed and validated
• Secrets are properly distributed and accessible
• Elasticsearch API is accessible on all nodes
• Inter-node communication works (port 9300)