TInsensitiveCaseKey: fix method signature

src/Collections/TInsensitiveCaseKey.php

@@ -8,8 +8,10 @@ namespace jrosset\Collections;
 trait TInsensitiveCaseKey {
     /**
      * @inheritDoc
+     *
+     * @noinspection PhpMissingReturnTypeInspection
      */
-    protected function _normalizeKey (int|string $key): int|string {
+    protected function _normalizeKey ($key) {
         return mb_strtolower($key);
     }
 }