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/tailscale/cli/cli_test.go

@@ -962,8 +962,8 @@ func TestPrefFlagMapping(t *testing.T) {
 	}
 
 	prefType := reflect.TypeFor[ipn.Prefs]()
-	for i := range prefType.NumField() {
-		prefName := prefType.Field(i).Name
+	for field := range prefType.Fields() {
+		prefName := field.Name
 		if prefHasFlag[prefName] {
 			continue
 		}