[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",
  ]

[Tofandel]

We only need array_merge, given that rules are always nested only 2 levels deep and are always associative arrays (Asserted by the ValidationRuleParser)

[taylorotwell]

Any reason this was sent to master vs. 11.x?

[driesvints]

@taylorotwell as we suspected in the past this might be a breaking change: #39368

[Tofandel]

I'm arguing it's not a breaking change, because validators requires keyed arrays and so having numeric keys before was not possible without encountering this bug (and thus make the validation always fail/unusable) and it would not cause any change with assoc arrays either anyways, only with non sequential numeric keys

The previous PR was about changing the behavior to array_replace_recursive instead of array_merge_recursive which was a whole level of a breaking change, but this PR keeps the merge while fixing the numeric key issue. I've been told to submit to master anyways but I really think there is no breaking change here

Whether you want to err on the side of extreme caution or not is up to you, me I would just like to see this bug fixed sometime in the future but it doesn't matter major or minor