Valid Voices for Alexa are Not Current

The list of valid Alexa voices [here](https://github.com/speechmarkdown/speechmarkdown-js/blob/59060f361e3efc1cc04e8c22e67294009d251081/src/formatters/GoogleAssistantSsmlFormatter.ts#L13) is no longer current with Alexa's [supported voices](https://developer.amazon.com/de-DE/docs/alexa/custom-skills/speech-synthesis-markup-language-ssml-reference.html#supported-voices).