testing/librespot: upgrade to 0.4.1

This upgrades librespot to 0.4.1, and updates dependencies to reflect MSRV bump from upstream.

Release notes since 0.3.1:
https://github.com/librespot-org/librespot/releases/tag/v0.4.0
https://github.com/librespot-org/librespot/releases/tag/v0.4.1