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.go

@@ -352,8 +352,7 @@ func main() {
 			// If there's nothing to retry and no non-retryable tests have
 			// failed then we've probably hit a build error.
 			if err := <-runErr; len(toRetry) == 0 && err != nil {
-				var exit *exec.ExitError
-				if errors.As(err, &exit) {
+				if exit, ok := errors.AsType[*exec.ExitError](err); ok {
 					if code := exit.ExitCode(); code > -1 {
 						os.Exit(exit.ExitCode())
 					}