Update posts when issue/PR changes #11

Would be handy to at least handle state changes like open/closed (e.g. change the color to purple when closed), but it would also be nice to handle title/description updates to things are reflected properly.

Would require tracking state somehow; I wonder if there's a way to use the GitHub Actions cache for this? I would not want to require repos that use this action to keep a file or branch in their project repo for the state…

It's probably okay if older messages are not updated (e.g. if we rely on the 7-day retention of GitHub Actions artifacts).