Adds explicit mention of Valkey in the README.

[stockholmux]

A Valkey community member had questions about using this module with Valkey instead of Redis, this contribution aims directly answer this question.

Since both Valkey 7.2 and Redis 7.2 have the same API and and this repo has testing for Valkey, this change adds an explicit mention of Valkey in the README along side Redis.

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[mp911de]

Hey Kyle, thanks for reaching out. Our strategy for now is to support Valkey on a best-effort basis.

[stockholmux]

thanks @mp911de !