Removes "mixed" type

Author: jtreminio

modules/openapi-generator/src/main/resources/php/ObjectSerializer.mustache

@@ -628,7 +628,7 @@ class ObjectSerializer
      * credit: https://github.com/FranBar1966/FlatPHP
      */
     private static function flattenArray(
-        mixed $source,
+        $source,
         array &$destination,
         string $start = ''
     ) {

samples/client/petstore/php/OpenAPIClient-php/lib/ObjectSerializer.php

@@ -637,7 +637,7 @@ public static function buildQuery(array $params, $encoding = PHP_QUERY_RFC3986):
      * credit: https://github.com/FranBar1966/FlatPHP
      */
     private static function flattenArray(
-        mixed $source,
+        $source,
         array &$destination,
         string $start = ''
     ) {

samples/client/petstore/php/psr-18/lib/ObjectSerializer.php

@@ -637,7 +637,7 @@ public static function buildQuery(array $params, $encoding = PHP_QUERY_RFC3986):
      * credit: https://github.com/FranBar1966/FlatPHP
      */
     private static function flattenArray(
-        mixed $source,
+        $source,
         array &$destination,
         string $start = ''
     ) {