fix(php): Deprecation warning fix for PHP 8.1+ (#3562)

Co-authored-by: shortcuts <[email protected]>

Author: aykutersoy

templates/php/model_generic.mustache

@@ -346,7 +346,7 @@ class {{classname}} extends \Algolia\AlgoliaSearch\Model\AbstractModel {{^parent
      *
      * @return boolean
      */
-    public function offsetExists($offset)
+    public function offsetExists($offset): bool
     {
         return isset($this->container[$offset]);
     }
@@ -358,7 +358,7 @@ class {{classname}} extends \Algolia\AlgoliaSearch\Model\AbstractModel {{^parent
      *
      * @return mixed|null
      */
-    public function offsetGet($offset)
+    public function offsetGet($offset): mixed
     {
         return $this->container[$offset] ?? null;
     }
@@ -371,7 +371,7 @@ class {{classname}} extends \Algolia\AlgoliaSearch\Model\AbstractModel {{^parent
      *
      * @return void
      */
-    public function offsetSet($offset, $value)
+    public function offsetSet($offset, $value): void
     {
         if (is_null($offset)) {
             $this->container[] = $value;
@@ -387,7 +387,7 @@ class {{classname}} extends \Algolia\AlgoliaSearch\Model\AbstractModel {{^parent
      *
      * @return void
      */
-    public function offsetUnset($offset)
+    public function offsetUnset($offset): void
     {
         unset($this->container[$offset]);
     }