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/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