`CHANGELOG` check in CI doesn't check for deleted release sections

[ErichDonGubler]

Deleting an entire released section has no consequences with the current CHANGELOG check, i.e.:

 ## Unreleased
 
 hay sup

-## Release 2
-
-later suckers
-
 ## Release 1
 
 - i'm freeeee

It's unlikely to pass review, but it'd be nice to plug this gap.

[teddygood]

I'd like to work on this. Could you assign it to me?

[ErichDonGubler]

@teddygood: Have at it! 🙂