all: use Go 1.22 range-over-int

Updates #11058

Change-Id: I35e7ef9b90e83cac04ca93fd964ad00ed5b48430
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

util/dnsname/dnsname.go

@@ -40,7 +40,7 @@ func ToFQDN(s string) (FQDN, error) {
 	}
 
 	st := 0
-	for i := 0; i < len(s); i++ {
+	for i := range len(s) {
 		if s[i] != '.' {
 			continue
 		}

util/dnsname/dnsname_test.go

@@ -224,7 +224,7 @@ func BenchmarkToFQDN(b *testing.B) {
 	for _, test := range tests {
 		b.Run(test, func(b *testing.B) {
 			b.ReportAllocs()
-			for i := 0; i < b.N; i++ {
+			for range b.N {
 				sinkFQDN, _ = ToFQDN(test)
 			}
 		})