On microbit-v2, getting .well-known/core would otherwise result in a
stack overflow.
Consequently, some boards were removed from the list of supported boards
as the currently required RAM exceeds their capacity.
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.