docs(admin): list Redis 4.0+ in system requirements - #15736

Summary

Closes #11030

System requirements listed OS / DB / webserver / PHP but not the recommended Redis version for caching and locking. Add an optional Cache / locking row: Redis Open Source 4.0+ (matches caching_configuration.rst), plus Valkey/KeyDB and APCu, with a link to the caching guide.

Test plan

• Docs build for admin_manual/installation/system_requirements.rst
• Table row links to caching configuration
• Minimum Redis version aligns with caching docs (4.0+)

AI disclosure

This PR was drafted with the help of AI coding assistants (Cursor agents / LLM-based tools), including the description. I am responsible for it and happy to rework anything that doesn't fit.

All commits carry an Assisted-by: Cursor:grok-4.7 trailer.