[Variant] Rename `variant_experimental` flag to `variant` and remove warnings about being experimental

[alamb]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.
Part of #6736

Once we are happy with the state of variant support, we should rename the variant flag and update the comments to reflect it is no longer experimental.

Describe the solution you'd like
Rename variant_experimental flag to variant and remove warnings about being experimental

Describe alternatives you've considered

Additional context