rustfmt: update expected Rust edition

In this case it doesn't change anything, but we've been using 2021 for a
while now.

tests/test-check-rust-format.t

@@ -7,5 +7,5 b''
 Warning: Keep this in sync with hghave.py
   $ RUSTFMT=$(rustup which --toolchain nightly-2024-07-16 rustfmt)
   $ for f in `testrepohg files 'glob:**/*.rs'` ; do
-  >   $RUSTFMT --check --edition=2018 --unstable-features --color=never $f
+  >   $RUSTFMT --check --edition=2021 --unstable-features --color=never $f
   > done