redis-ha: fix busybox-ash-ism in redis_liveness.sh

[bootc]

What this PR does / why we need it:

The ${response:0:7} syntax is non-POSIX and not understood by Debian's dash shell, so if a Debian-based Redis image is used the liveness check breaks on slaves. A case statement makes this check more legible anyway, in my opinion, and works in all the POSIX-compatible shells (busybox ash, dash, and bash).

Which issue this PR fixes

No issue created (yet).

Checklist

• [x] DCO signed
• [x] Chart Version bumped
• (n/a) Variables are documented in the README.md
• [x] Title of the PR starts with chart name (e.g. [stable/mychartname])