"redis" -> "valkey" docs misc changes

[rukai]

Covers most of the remaining changes:

• Adds a changelog entry for shotover 0.6
• A bunch of docs fixes
• Fix feature change missed in rename cargo feature "redis" -> "valkey" #1836 , it seems like a bug in cargo hack that this was able to slip through, I've raised an issue here --feature-powerset --at-least-one-of silently drops unknown features taiki-e/cargo-hack#259

Remaining work:

• Update example docs in this repo
• rename redis-cache directory - rename redis-cache dir -> valkey-cache #1848
• Update external example repos - TODO after release of shotover 0.6

[codspeed-hq]

CodSpeed Performance Report

Merging #1847 will not alter performance

_{Comparing rukai:redis_valkey_docs_misc (2137431) with main (71d1c7f)}

Summary

✅ 38 untouched benchmarks