redis-ha: fix readiness probe for non-connected slaves

[bootc]

What this PR does / why we need it:

When Redis is running as a slave, don't pass readiness probes until and unless the replica is fully synchronised.

This resolves the issue where, when Redis is upgraded, the replica is marked ready while still syncing from the master. This can lead to the master pod being terminated and restarted with no slaves ready to take over from it, leaving the Redis cluster failed until the original primary comes back up again.

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])