This commit updates the documentation for the unified installer. - The Open Source example is replaced with version 2025.1 (Source Available, currently supported, LTS). - The info about CentOS 7 is removed (no longer supported). - Java 8 is removed. - The example for cassandra-stress is removed (as it was already removed on other installation pages). Fixes https://github.com/scylladb/scylladb/issues/28150 Closes scylladb/scylladb#28152