Fix `base64` dependency error

[notmandatory]

This looks like a new error that just popped up in the master and release/0.28 branches.

error: package base64 v0.21.1 cannot be built because it requires rustc 1.60.0 or newer, while the currently active rustc version is 1.57.0

see: https://github.com/bitcoindevkit/bdk/actions/runs/5018045314/jobs/9067260985?pr=957

[Sosthene00]

It seems that base64 updated it's MSRV 2 days ago: marshallpierce/rust-base64#238

[notmandatory]

Thanks @Sosthene00 , @danielabrozzoni is working on a fix for the master branch to pin the version:
base64 = "=0.21.0". I'll do a corresponding PR for the release/0.28 branch.

[notmandatory]

Root cause is that rust-base64 project changed their MSRV in patch release 0.21.1 from 1.57.0 to 1.60.0. I wish projects wouldn't do that but it happens.