all: use Go 1.26 things, run most gofix modernizers

I omitted a lot of the min/max modernizers because they didn't
result in more clear code.

Some of it's older "for x := range 123".

Also: errors.AsType, any, fmt.Appendf, etc.

Updates #18682

Change-Id: I83a451577f33877f962766a5b65ce86f7696471c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnlocal/extension_host_test.go

@@ -1010,9 +1010,8 @@ func TestNilExtensionHostMethodCall(t *testing.T) {
 	t.Parallel()
 
 	var h *ExtensionHost
-	typ := reflect.TypeOf(h)
-	for i := range typ.NumMethod() {
-		m := typ.Method(i)
+	typ := reflect.TypeFor[*ExtensionHost]()
+	for m := range typ.Methods() {
 		if strings.HasSuffix(m.Name, "ForTest") {
 			// Skip methods that are only for testing.
 			continue