[base64] Use the new 0.20 version of Rust's base64 crate, which is now non-malleable

[kchalkias]

Consider replacing base64ct with the faster (but not constant time) base64 Rust crate when possible.
Note that the constant-time requirement is still important when encoding/decoding confidential data (ie secret seeds, private keys / key pairs etc)

reference: https://twitter.com/kostascrypto/status/1601967338970091520?s=20&t=HL_vuR2jlJcN3eBsRApSQQ