[12.x] Preserve numeric keys on the first level of the validator rules

[Tofandel]

Fixes #51499 without changing the merge behavior (see testMergeRules in the PR that #39368 wouldn't have passed), simply preserve all the keys passed to the validator rules

Bug description

Running the following in a tinker session

Validator::validate(['foo' => 'baz', 4 => 'foo', 8 => 'baz'], ['foo' => 'required', 4 => 'required', 8 => 'required']);

Results in

   Illuminate\Validation\ValidationException  The 0 field is required. (and 1 more error).

Because array_merge_recursive rekeys any numeric keys it encounters

array_merge_recursive([], ['foo' => 'required', 4 => 'required', 8 => 'required']);
= [
    "foo" => "required",
    0 => "required",
    1 => "required",
  ]