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.

We have updated the library used for accessing B2 to remove this warning.

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