fix: avn service cli also support Valkey

[TibsAtWork]

Describe your changes

avn service cli also supports connecting to an Aiven for Valkey service.

It connects to Valkey or Redis using whichever of valkey-cli or redis-cli is available (validated by trying it out!), but I think it's more useful to reference valkey-cli (only) in the text (it does say "such as"), especially as that will make it simpler to remove mention of Aiven for Caching when that is retired.

Checklist

• The first paragraph of the page is on one line.
• The other lines have a line break at 90 characters.
• I checked the output.
• I applied the style guide.
• My links start with /docs/.

[staceysalamon-aiven]

Thanks for doing this!