Switch Redis to Valkey in deploy template and tests

[jjatinggoyal]

As mentioned by @dhh in https://github.com/basecamp/kamal/pull/1044#issuecomment-2394796495, this PR replaces Redis with Valkey (the FOSS fork of Redis) in deploy template and tests.

The deploy template now uses valkey/valkey:8 image for cache, which is the latest release with performance improvements over Valkey 7 / Redis 7, and still retains full compatibility with Redis OSS 7.2, which means the current ruby adapter for redis remains compatible.