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>

clientupdate/clientupdate.go

@@ -1292,6 +1292,6 @@ func requireRoot() error {
 }
 
 func isExitError(err error) bool {
-	var exitErr *exec.ExitError
-	return errors.As(err, &exitErr)
+	_, ok := errors.AsType[*exec.ExitError](err)
+	return ok
 }

clientupdate/clientupdate_test.go

@@ -451,7 +451,7 @@ func TestSynoArch(t *testing.T) {
 			synoinfoConfPath := filepath.Join(t.TempDir(), "synoinfo.conf")
 			if err := os.WriteFile(
 				synoinfoConfPath,
-				[]byte(fmt.Sprintf("unique=%q\n", tt.synoinfoUnique)),
+				fmt.Appendf(nil, "unique=%q\n", tt.synoinfoUnique),
 				0600,
 			); err != nil {
 				t.Fatal(err)