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>

tool/gocross/exec_other.go

@@ -21,8 +21,7 @@ func doExec(cmd string, args []string, env []string) error {
 
 	// Propagate ExitErrors within this func to give us similar semantics to
 	// the Unix variant.
-	var ee *exec.ExitError
-	if errors.As(err, &ee) {
+	if ee, ok := errors.AsType[*exec.ExitError](err); ok {
 		os.Exit(ee.ExitCode())
 	}