This restores compatibility with current nightlies by using the latest
coap-message version (after a nightly-only feature that uses saw a late
change), and also removes obstacles for use with stable Rust 1.59 (in
unrelated parts; the CoAP example is still nightly-only).
Contributes-To: https://github.com/RIOT-OS/RIOT/issues/17526
This pulls in adjustments to the C-nonbreaking but Rust-breaking API
change in https://github.com/RIOT-OS/RIOT/pull/17359
cstr_core is forcibly kept at 0.2.4 because 0.2.5 needs a newer nightly
than riotdocker currently has.