The change in 399e25cc was did not have the intended effect: As the
local crates still all defined 0.7 as the riot-wrappers version, that
dependency was actually down- rather than upgraded, and thus did not
effect the stabilizations.
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
