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>

net/dns/wsl_windows.go

@@ -172,8 +172,7 @@ func (fs wslFS) Truncate(name string) error { return fs.WriteFile(name, nil, 064
 
 func (fs wslFS) ReadFile(name string) ([]byte, error) {
 	b, err := wslCombinedOutput(fs.cmd("cat", "--", name))
-	var ee *exec.ExitError
-	if errors.As(err, &ee) && ee.ExitCode() == 1 {
+	if ee, ok := errors.AsType[*exec.ExitError](err); ok && ee.ExitCode() == 1 {
 		return nil, os.ErrNotExist
 	}
 	return b, err