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>

cmd/testwrapper/testwrapper_test.go

@@ -273,8 +273,7 @@ func TestCached(t *testing.T) {}
 }
 
 func errExitCode(err error) (int, bool) {
-	var exit *exec.ExitError
-	if errors.As(err, &exit) {
+	if exit, ok := errors.AsType[*exec.ExitError](err); ok {
 		return exit.ExitCode(), true
 	}
 	return 0, false