Match MSRV to upstream, 1.64 -> 1.73

[cpu]

This crate's MSRV is currently 1.64:

rustls-ffi/Cargo.toml

Line 12 in 594dc1e

rust-version = "1.64"

The upstream Rustls ecosystem (rustls, webpki, platform-verifier, etc) are moving to 1.73 and we should follow: rustls/rustls#2220