Update deprecated field in rustfmt.toml

This fixes the "Warning: the `version` option is deprecated. Use `style_edition` instead." that we see when running `cargo +nightly fmt`.

The 2021 edition is the last one matching the version "One".

rustfmt.toml

@@ -53,7 +53,7 @@ match_block_trailing_comma = false
 blank_lines_upper_bound = 1
 blank_lines_lower_bound = 0
 edition = "2018"
-version = "One"
+style_edition = "2021"
 inline_attribute_width = 0
 format_generated_files = true
 merge_derives = true