fixed Announcement attributes to match docs

[bcwood]

Fixes #86

See: https://docs.joinmastodon.org/entities/Announcement/

I did not change the property names in the .NET class since that would be a breaking change.

[glacasa]

Thanks for the fix ! Looks like the doc was incorrect when I wrote it (https://github.com/mastodon/documentation/blob/69e9017730131a3fbd2d92e65ea2a3de793fa0fc/content/en/entities/announcement.md)

I'm renaming them, and marking the incorrect property names as Obsolete, it was added recently and I think it's still time to change it without breaking lots of code.

[bcwood]

Sounds like a good plan! I had wondered about the documentation being incorrect at some point. I noticed that the anchor link for published_at is still pointing to created_at, so I thought that might be the case.