[12.x] Fix Accept header cache invalidation when header is modified

[faisuc]

When modifying the Accept header in middleware using $request->headers->set('Accept', 'application/json'), methods like wantsJson(), acceptsJson(), and prefers() would return stale values because Symfony's getAcceptableContentTypes() caches the result and doesn't clear it when headers are changed.

This caused issues in middleware scenarios like:

if ($request->acceptsAnyContentType()) {
    $request->headers->set('Accept', 'application/json');
}
if (!$request->wantsJson()) {
    return response()->json(['message' => 'Not Acceptable.'], 406);
}

Even after setting the Accept header to application/json, wantsJson() would still return false because it was using the cached value from before the header change.

Fixes: #57868