Make it known that get-k8s-versions is only relevant when using localkube

[patrikerdes]

Added a reference to localkube in the help text and in the output from
the command.

(Also removed the redundant word "available" and changed the capitalization of Kubernetes.)

[minikube-bot]

Can one of the admins verify this patch?

[patrikerdes]

Somewhat related to #2156

[codecov-io]

Codecov Report

Merging #2204 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #2204   +/-   ##
=======================================
  Coverage   27.73%   27.73%           
=======================================
  Files          82       82           
  Lines        5538     5538           
=======================================
  Hits         1536     1536           
  Misses       3806     3806           
  Partials      196      196

Impacted Files	Coverage Δ
cmd/minikube/cmd/get_kubernetes_versions.go	40% <ø> (ø)	⬆️
...inikube/kubernetes_versions/kubernetes_versions.go	65.3% <100%> (ø)	⬆️

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[r2d4]

Thanks! This makes things a lot clearer during the switch.