Bugfix: Improve HTTP/2 support for REST backend

If `rest-server` tried to gracefully shut down an HTTP/2 connection still in
use by the client, it 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 issue has now been resolved.

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