Upgrade base64 to 0.21.7 am: 86d8e5b747 am: 81ef64c5ce

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/base64/+/2939820

Change-Id: I094842c9b7ff27dc08a29689a319618c559f2b99
Signed-off-by: Automerger Merge Worker <[email protected]>