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>

client/local/local.go

@@ -192,8 +192,8 @@ func (e *AccessDeniedError) Unwrap() error { return e.err }
 
 // IsAccessDeniedError reports whether err is or wraps an AccessDeniedError.
 func IsAccessDeniedError(err error) bool {
-	var ae *AccessDeniedError
-	return errors.As(err, &ae)
+	_, ok := errors.AsType[*AccessDeniedError](err)
+	return ok
 }
 
 // PreconditionsFailedError is returned when the server responds
@@ -210,8 +210,8 @@ func (e *PreconditionsFailedError) Unwrap() error { return e.err }
 
 // IsPreconditionsFailedError reports whether err is or wraps an PreconditionsFailedError.
 func IsPreconditionsFailedError(err error) bool {
-	var ae *PreconditionsFailedError
-	return errors.As(err, &ae)
+	_, ok := errors.AsType[*PreconditionsFailedError](err)
+	return ok
 }
 
 // bestError returns either err, or if body contains a valid JSON
@@ -1071,7 +1071,7 @@ func tailscaledConnectHint() string {
 	// ActiveState=inactive
 	// SubState=dead
 	st := map[string]string{}
-	for _, line := range strings.Split(string(out), "\n") {
+	for line := range strings.SplitSeq(string(out), "\n") {
 		if k, v, ok := strings.Cut(line, "="); ok {
 			st[k] = strings.TrimSpace(v)
 		}

client/systray/logo.go

@@ -233,8 +233,8 @@ func (logo tsLogo) renderWithBorder(borderUnits int) *bytes.Buffer {
 		dc.InvertMask()
 	}
 
-	for y := 0; y < 3; y++ {
-		for x := 0; x < 3; x++ {
+	for y := range 3 {
+		for x := range 3 {
 			px := (borderUnits + 1 + 3*x) * radius
 			py := (borderUnits + 1 + 3*y) * radius
 			col := fg