[php-slim4] Update .htaccess template

Created by: ybelenko

Just found more enhanced .htaccess in Slim 4 skeleton. Ref: https://github.com/slimphp/Slim-Skeleton/blob/037cfa2b6885301fc32a5b18a00a251a534aac81/public/.htaccess

Also checked it locally(HTTP 501 response is totally fine and expected with fresh build):

$  curl --request GET \
>   --url http://phpslim4.test/v2/pet/5 \
>   --header 'api_key: apiKey' \
>   --header 'x-openapiserver-mock: ping' \
>   --header 'Accept: application/json'
{
    "message": "501 Not Implemented",
    "exception": [
        {
            "type": "Slim\\Exception\\HttpNotImplementedException",
            "code": 501,
            "message": "How about extending AbstractPetApi by OpenAPIServer\\Api\\PetApi class implementing getPetById as a GET method?",
            "file": "/Users/ybelenko/Sites/openapi-generator/samples/server/petstore/php-slim4/lib/App/RegisterRoutes.php",
            "line": 844
        }
    ]
}

cc @jebentier, @dkarlovi, @mandrean, @jfastnacht, @renepardon

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