internal/http3: prevent Server handler from writing longer body than declared

Like in HTTP/1 and HTTP/2, this CL changes the Server handler so that
Write will return an error and automatically trim the content being
written, when attempting to write a body content that is longer than
what has been declared in the "Content-Length" header.

For golang/go#70914

Change-Id: I638efb8ec96f926f86c389f3cac79f1b38a5b36b
Reviewed-on: https://go-review.googlesource.com/c/net/+/748681
Reviewed-by: Nicholas Husin <husin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Damien Neil <dneil@google.com>