rust: we are way beyond 2.53

rust: we are way beyond 2.53

Earlier we timelined that we'd tune our build procedures to build
with Rust by default in Git 2.53, but we are already in prerelease
freeze for 2.54 now.  Update the BreakingChanges document to delay
it until Git 2.55 (slated for the end of June 2026).

Noticed-by: brian m. carlson <sandals@crustytoothpaste.net>
Helped-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>