Add rules while preserving integer rule keys inside the validator

[jeffreyzant]

This resolves #39362.

[taylorotwell]

Could be a breaking change for some users.

[Tofandel]

I don't think array_replace_recursive is the right change instead we need a custom merge function, because array_merge_recursive appends ANY numeric key to the array, it doesn't check if the array is an assoc array first instead of preserving the keys, and in the case of a validator, it doesn't make sense to pass in an associative array

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

I also don't think a small potential breaking change warrants not fixing this bug, because it's quite offputting behavior (and the fact that it's broken means it already cannot be used like this, so I don't see what kind of code it would break), and if it's really a problem it can go in a major (which this PR was intended for, but I do agree changing to array_replace_recursive was a breaking change and not the right solution)