Change: Update dependencies, require Go >= 1.13

Restic now requires Go to be at least 1.13. This allows simplifications in the
build process and removing workarounds.

This is also probably the last version of restic still supporting mounting
repositories via fuse on macOS. The library we're using for fuse does not
support macOS any more and osxfuse is not open source any more.

https://github.com/restic/restic/pull/2600
https://github.com/restic/restic/pull/2852
https://github.com/restic/restic/pull/2927
https://github.com/bazil/fuse/issues/224
https://github.com/osxfuse/osxfuse/issues/590