Add deprecation notice to fluxctl and Flux v1 Helm chart

[stefanprodan]

To prevent new users to start with v1 instead of v2, we could add a warning to both fluxctl install and to helm install flux.
For the CLI, the warning should be written to stderr to not affect the install command, for Helm the warning could be place in NOTES.txt.

The deprecation notice must include a link to Flux v2 get started guide.