Warn if using func v2 or when creating a v2 function app

[anthonychu]

Encourage folks to move to v3.

• warn if func is still v2
• warn if creating a functionapp locally or in Azure that's v2
• warn if deploying to a v2 app and point users to this announcement: Platform upgrade for Azure Functions v2 apps coming in October 2020 Azure/app-service-announcements#266

/cc @pragnagopa

[fiveisprime]

Need to work with the Functions team to define the telemetry required to track how customers are interacting with the warning.

[fiveisprime]

• Recommend customers upgrade to V3
• Don't migrate exisitng V2 apps to V3
• New apps get V3

For Linux: need feedback from @pragnagopa on the deployment settings to make sure V2 apps run correctly with a V3 runtime.

[pragnagopa]

Same warning/recommendation applies for apps running on Linux.

@fiveisprime - Do let me know which deployment settings you are referring to.