Drop compatibility with EOL Kubernetes versions

Description

Simplifies chart by requiring Kubernetes 1.20+ and dropping all outdated versions
Changes defaults for persistent volumes to only check permissions on the root, which will speed start-ups for the vast majority of folks

Possible challenges

This is a breaking change - if you try and install the chart on Kubenetes 1.19 or earlier, it will fail validation

Checklist

[x] Chart version bumped in Chart.yaml
[x] Any new variables have documentation and examples in values.yaml, even if commented out
[x] Any new variables added to the README.md
[x] ...additional commit added for CHANGELOG.md entry
[x] Helm lint + tests passing?

gocd / helm-chart