Refine experimental tag message

[arrownj]

Is your feature request related to a problem? Please describe.
No

Describe the solution you'd like
Currently the experimental tag message is:

Command group 'local-context' is experimental and not covered by customer support. Please use with discretion.

which calls out that customer support is limited and might turn people away from trying it.

PM gives some other suggestions, they are as follows:

This command group is experimental. Customer support is not available for preview features. We recommend that you back up your work frequently

or

This command group is experimental and we would love for you to try out the experience and share us your feedback here: aka.ms/cli_feedback

Describe alternatives you've considered
No

Additional context
No

[arrownj]

Update experimental message in Knack.

[jiasli]

Knack is a general CLI framework. I am not sure if limiting the error message to Azure CLI is a good idea.

[jiasli]

Will be covered by #15730 and microsoft/knack#223. The experimental message will be made configurable.