minor #43903 Fix more generic types (derrabus)

This PR was merged into the 5.4 branch.

Discussion
----------

Fix more generic types

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | N/A
| License       | MIT
| Doc PR        | N/A

Some changes I wanted to push to #42471, but then it got merged already. 😇

Commits
-------

c54d61e9af Fix more generic types

Author: fabpot

HeaderBag.php

@@ -16,15 +16,15 @@
  *
  * @author Fabien Potencier <fabien@symfony.com>
  *
- * @implements \IteratorAggregate<string, array<int, string|null>>
+ * @implements \IteratorAggregate<string, list<string|null>>
  */
 class HeaderBag implements \IteratorAggregate, \Countable
 {
     protected const UPPER = '_ABCDEFGHIJKLMNOPQRSTUVWXYZ';
     protected const LOWER = '-abcdefghijklmnopqrstuvwxyz';
 
     /**
-     * @var array<string, array<int, string|null>>
+     * @var array<string, list<string|null>>
      */
     protected $headers = [];
     protected $cacheControl = [];
@@ -255,7 +255,7 @@ public function removeCacheControlDirective(string $key)
     /**
      * Returns an iterator for headers.
      *
-     * @return \ArrayIterator<string, array<int, string|null>>
+     * @return \ArrayIterator<string, list<string|null>>
      */
     #[\ReturnTypeWillChange]
     public function getIterator()