chore: Switch to dates and modern naming conventions

[scottlovegrove]

This PR basically applies my feedback from #351. Because the package has gone out containing the changes from #351, I've had to this in a deprecated manner to ensure we aren't introducing a breaking change.

[amix]

Thanks, @scottlovegrove 👏

2 macro comments:

• I would recommend not implementing backward compatibility. We just introduced this yesterday, and we have an opportunity to get this right without creating debt 🤔
• I find some of the tests too verbose. Maybe we could just for the critical things (not tirival things like date formatting)

[amix]

What were we smoking here when initiatorIdNull was created 😱🤯

[scottlovegrove]

• I would recommend not implementing backward compatibility. We just introduced this yesterday, and we have an opportunity to get this right without creating debt 🤔

I considered doing this, but I looked on npm, and it's already been downloaded nearly 200 times. Now that could be just our own CI checks, I don't know, but it seemed a not-insignificant amount, and I didn't want to be a bad SDK maintainer :)