Enhancement: Enable HTTP/2 for backend connections

Go's HTTP library usually automatically chooses between HTTP/1.x and HTTP/2
depending on what the server supports. But for compatibility this mechanism
is disabled if DialContext is used (which is the case for restic). This change
allows restic's HTTP client to negotiate HTTP/2 if supported by the server.

https://github.com/restic/restic/pull/3254
