Update default Istio version

[thesuperzapper]

As of deployKF 0.1.4, we ship Istio 1.17.3, and in deployKF 0.1.5 we plan to update to at least the last version of 1.17 (which is 1.17.8). These versions are very old, and we don't want to encourage using EOL Istio versions.

While deployKF supports istio <=1.20 (and probably 1.21 and 1.22 too), and users can bring their own istio, or simply change the version in the values, we should update the default.

Since Istio does not support their releases for very long, this is probably something we will have to do regularly (2-3 times a year).

We need to consider the following things when updating:

1. Existing users will need to restart all their Pods:
   • (so the Istio webhook injects the new sidecar container image tag)
   • We need to provide a script to do this automatically which will be non-trivial to do safely. DONE docs: add update_istio_sidecars.sh script #132
2. It's not 100% clear that older versions of Kubernetes will work properly with extremely new Istio versions.
   • We need to confirm this, and warn users in the kubernetes version table if they need to use an older istio version.
   • Note, the official Istio support matrix is quite conservative in my experience, so it's not necessarily true that new istio does not support older Kubernetes.

[thesuperzapper]

Here are the breaking changes for all minor versions newer than 1.17 (our current version in deployKF 0.1.4):

• https://istio.io/latest/news/releases/1.18.x/announcing-1.18/upgrade-notes/
• https://istio.io/latest/news/releases/1.19.x/announcing-1.19/upgrade-notes/
• https://istio.io/latest/news/releases/1.20.x/announcing-1.20/upgrade-notes/
• https://istio.io/latest/news/releases/1.21.x/announcing-1.21/upgrade-notes/