crypto: use clear built-in

Replace for loops with clear built-in, available since Go 1.21.

Change-Id: I16a2691a68042e9c5cd9bc4197690fa541a081eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/704877
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Mark Freeman <markfreeman@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

conn.go

@@ -228,9 +228,7 @@ func (hc *halfConn) changeCipherSpec() error {
 	hc.mac = hc.nextMac
 	hc.nextCipher = nil
 	hc.nextMac = nil
-	for i := range hc.seq {
-		hc.seq[i] = 0
-	}
+	clear(hc.seq[:])
 	return nil
 }
 
@@ -239,9 +237,7 @@ func (hc *halfConn) setTrafficSecret(suite *cipherSuiteTLS13, level QUICEncrypti
 	hc.level = level
 	key, iv := suite.trafficKey(secret)
 	hc.cipher = suite.aead(key, iv)
-	for i := range hc.seq {
-		hc.seq[i] = 0
-	}
+	clear(hc.seq[:])
 }
 
 // incSeq increments the sequence number.