support for base64 0.21

We would like to upgrade the rust-base64 package in Debian after the release of bookworm.

While we do have a mechanism for packaging multiple versions of a crate at the same time it is something we try to avoid if possible, so we would appreciate a fix to use current (0.21) base64.

See also: #961 (closed) where the dependency on base64 was limited to < 0.20 due to an incompatibility in the code.