Removal DynamicResponseModel

[malles]

After updating to the latest release I got errors concerning invalid property names. After investigation I learned that the DynamicResponseModel has been removed from the responses.
On it's own a good thing, it allows for better typing.

However the wiki still promotes it: https://github.com/ActiveCampaign/postmark-php/wiki/Getting-Started#about-responses-from-api-calls and I think this should be mentioned in an upgrade guide.