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

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
Part of https://github.com/apache/arrow-rs/issues/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**

Search the repository for `variant_experimental` and comments or warnings describing variant support as experimental. Rename the flag to `variant`, update the related wording, and run the relevant Rust checks or tests to confirm the renamed feature still builds and behaves correctly.

Written by the indexing model from the issue text.