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.go

@@ -339,7 +339,7 @@ func (h *ExtensionHost) FindMatchingExtension(target any) bool {
 
 	val := reflect.ValueOf(target)
 	typ := val.Type()
-	if typ.Kind() != reflect.Ptr || val.IsNil() {
+	if typ.Kind() != reflect.Pointer || val.IsNil() {
 		panic("ipnext: target must be a non-nil pointer")
 	}
 	targetType := typ.Elem()