[install_flux.sh] Addresses two warning messages observed when running the script

General MR

Summary

This addresses two warning messages that were popping up, and possibly causing people to ignore other information.

Flag --short has been deprecated, and will be removed in the future. The --short output will become the default.

and

Warning: resource namespaces/flux-system is missing the kubectl.kubernetes.io/last-applied-configuration annotation which is required by kubectl apply. kubectl apply should only be used on resources created declaratively by either kubectl create --save-config or kubectl apply. The missing annotation will be patched automatically.

Relevant logs/screenshots

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