Respect DO_NOT_TRACK as standardized alternative to DOTNET_CLI_TELEMETRY_OPTOUT

[khellang]

DO_NOT_TRACK is a proposed unified standard for opting out of telemetry for TUI/console apps.
It would be nice if the dotnet CLI followed the same standard. See https://consoledonottrack.com/

[fowl2]

It seems like this has been denied. #17472 (comment)

[Extarys]

A year and a half later, any update on using this? I feel like it's a standard now on many OSS.

[marcpopMSFT]

Coming back to this, we reviewed the original effort and it doesn't appear to have been picked up broadly so we're going to continue to hold off for now.