Add itertools v0.11 -> v0.12 bump to changelog

Author: ytmimi

CHANGELOG.md

@@ -52,6 +52,7 @@
 
 - Updating `dirs 4.0.0 -> 5.0.1` and `cargo_metadata 0.15.4 -> 0.18.0` [#6033] (https://github.com/rust-lang/rustfmt/issues/6033)
   - For reference, here's the [dirs v5 changelog](https://github.com/dirs-dev/dirs-rs/blob/main/README.md#5)
+- Updated [itertools v0.11 -> v0.12](https://github.com/rust-itertools/itertools/blob/v0.12.1/CHANGELOG.md#0120) [#6093](https://github.com/rust-lang/rustfmt/pull/6093)
 - Addressed clap deprecations output when running `cargo check --features clap/deprecated` [#6101](https://github.com/rust-lang/rustfmt/pull/6101)
 - Bumped bytecount `0.6.4` -> `0.6.8` to fix compilation issues with the `generic-simd` feature. See [bytecount#92] and [bytecount#93]