[9.x] Fix array to string conversion when using custom validation messages for size rules

[mateusjunges]

Description

This PR is a follow up on #36885. I've tried ealier this year, but i think i haven't described correctly the issues or used wrong examples and it was rejected, so i'll try it again. When using custom validation messages for size rules, if you define your custom validation messages in `resources/lang/en/validation.php, then laravel thrown an exception when accessing those messages:

This exception comes from the following line, while replacing the :message:

framework/src/Illuminate/Support/MessageBag.php

Line 268 in 96d2b48

return str_replace([':message', ':key'], [$message, $messageKey], $format);

I'm sending this to master because if some user currently uses the custom validation messages, then the response he gets when using $validator->errors() is the following:

{
    "file":[
        {
           "file":"The file must be between 20 and 50 kilobytes.",
           "string":"Some custom message here"
        }
    ]
}

and this PR changes it to:

{
    "file":[
        "Some custom message here"
    ]
}

So, this changes the way you access validation errors from $validator->errors()->toArray()['file'][0]['string'] to $validator->errors()->toArray()['file'][0] or $validator->errors()->first('file), which is a breaking change.

Steps to reproduce

• Define custom validation messages in resources/lang/en/validation.php:

'custom' => [
    'file' => [
        'between' => [
            'numeric' => 'The :attribute must be between :min and :max.',
            'file' => 'The :attribute must be between :min and :max kilobytes.',
            'string' => 'Some custom message here',
            'array' => 'The :attribute must have between :min and :max items.',
        ],
    ],
],

• Create a test route in web.php to test the validation message:

Route::get('/test', function() {
   return Validator::make(['file' => 'filename.png'], ['file' => 'between:20,50'])->errors()->first('file');
});

• Go to localhost:8000/test

[mateusjunges]

Here #36885 (comment) taylor said that "an email is never a file or a numeric. It's basically string". I agree and i really should have described it better. But, with the file attribute, you could have two or more places in your project that uses file sometimes as string (filename) and sometimes as a file (uploaded file). So, if you use custom validation messages, then you get the array to string conversion. That's why i'm submitting this PR again.