Bugfix: Improve HTTP2 support for rest backend

If rest-server tried to gracefully shut down an HTTP2 connection still used by the client,
this could result in the following error.

```
http2: Transport: cannot retry err [http2: Transport received Server's graceful shutdown GOAWAY] after Request.Body was written; define Request.GetBody to avoid this error
```

This has been fixed.

https://github.com/restic/restic/pull/5018
https://forum.restic.net/t/receiving-http2-goaway-messages-with-windows-restic-v0-17-0/8367