Switch hashbrown to use 2021 edition am: 39a7818c41

Original change: https://android-review.googlesource.com/c/platform/prebuilts/rust/+/2955337

Change-Id: Id2906d045f8d55affbe2bfe713187460d7adf36d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>