MINOR: [Docs] Update Rust implementation status for Parquet Variant in Documentation

[alamb]

Rationale for this change

The Rust implementation supports the Variant type: https://docs.rs/parquet/latest/parquet/variant/enum.Variant.html

What changes are included in this PR?

Update the supported matrix to reflect this fact

Are these changes tested?

N/A

Are there any user-facing changes?

Documentation only changes, no functional changes

[github-actions]

Thanks for opening a pull request!

If this is not a minor PR. Could you open an issue for this pull request on GitHub? https://github.com/apache/arrow/issues/new/choose

Opening GitHub issues ahead of time contributes to the Openness of the Apache Arrow project.

Then could you also rename the pull request title in the following format?

GH-${GITHUB_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

or

MINOR: [${COMPONENT}] ${SUMMARY}

See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 3 benchmarking runs that have been run so far on merge-commit 2581991.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details.

[alamb]

Thank you @thisisnic 🙏