Upgrade rust/crates/protobuf-codegen to 2.20.0 am: b7f1d00f9f am: 7896a791e9

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/protobuf-codegen/+/1543965

MUST ONLY BE SUBMITTED BY AUTOMERGER

Change-Id: I53e88dd57cc1e556e8c87c9212fcf7eb9ac88abc