http2: use net.ErrClosed

Use errors.Is(err, net.ErrClosed) instead of checking for a known
string. net.ErrClosed is available since Go 1.16, the current minimum
version in go.mod is Go 1.18.

For golang/go#4373

Change-Id: Id98771874434bae7d9c6d1d4d36fddb28822eb39
Reviewed-on: https://go-review.googlesource.com/c/net/+/583016
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

http2/server.go

@@ -732,11 +732,7 @@ func isClosedConnError(err error) bool {
 		return false
 	}
 
-	// TODO: remove this string search and be more like the Windows
-	// case below. That might involve modifying the standard library
-	// to return better error types.
-	str := err.Error()
-	if strings.Contains(str, "use of closed network connection") {
+	if errors.Is(err, net.ErrClosed) {
 		return true
 	}