Bugfix: Remove `b2_download_file_by_name: 404` warning from B2 backend

In some cases the B2 backend could print `b2_download_file_by_name: 404: :
b2.b2err` warnings. These are only debug messages and can be safely ignored.

Restic now uses an updated library for accessing B2, which removes the warning.

https://github.com/restic/restic/issues/3750
https://github.com/restic/restic/issues/4144
https://github.com/restic/restic/pull/4146
