Fix incorrect key name in rustfmt.toml (#1005)

rustfmt.toml's 'edition' key only controls parsing behavior. We should
leave this to the default (read from Cargo.toml) so that rustfmt's
parsing behavior always matches what our code is targeting. Formatting
behavior is controlled by this separate key, 'style_edition', which we
can change separately from the actual code edition. This will allow us
to take 2024 edition formatting changes separately from code behavior
changes, should we so choose.

Part of #288

Author: smalis-msft

rustfmt.toml

@@ -3,4 +3,4 @@
 
 newline_style="unix"
 use_field_init_shorthand=true
-edition="2021"
+style_edition="2021"