Fix: Use strpos() instead of str_contains() for PHP 7.4 Comatibility

I was implementing this library in LearnDash Core and our CI checks for
PHP Compatibility going back to PHP 7.4. This was the only issue that
was flagged and it is a simple fix, so I figure it would be worthwhile
swapping this out.

Author: d4mation

src/Actions/DisplayNoticesInAdmin.php

@@ -141,7 +141,7 @@ private function passesScreenConditions(AdminNotice $notice): bool
                 }
             } elseif (is_string($condition)) {
                 // do a string comparison on the current url
-                if (str_contains($currentUrl, $condition)) {
+                if (strpos($currentUrl, $condition) !== false) {
                     return true;
                 }
             } else {