Calculation/DateTime Failure With PHP8 (#1661)

The following code generates an error with PHP8:
  if ($t1[1] > 29) {
    $t1[1] += 1900;

Under the "right" conditions, PHP8 evaluates the condition as true
when PHP8 is a non-numeric string and then generates an error trying
to perform += on that non-numeric string. Adding a numeric test
eliminates the problem. All unit tests involving this code now
succeed with both PHP7 and PHP8.

src/PhpSpreadsheet/Calculation/DateTime.php

@@ -489,7 +489,7 @@ class DateTime
             if ($yearFound) {
                 array_unshift($t1, 1);
             } else {
-                if ($t1[1] > 29) {
+                if (is_numeric($t1[1]) && $t1[1] > 29) {
                     $t1[1] += 1900;
                     array_unshift($t1, 1);
                 } else {