fix(perf): Fix transaction setter on scope to use containing_transaction

[sl0thentr0py]

fixes #1155, not sure why we have this discrepancy between getter and setter behaviour in the first place

[lobsterkatie]

I think it's fine to change this, but the entire business of getter and setter handling two different types is a headache which I'd love to see us fix. (It would take a new major, since it'd be a breaking change, but it's never led to anything but confusion, for us and users, so highly worth it, IMHO.)

So while it's true that this would make what the issue creator is doing work, IMHO it's kind of an antipattern which we should remove from the docs even if we don't remove from the code. scope.transaction.name = whatever will do it just fine, and doesn't rely on the weird legacy behavior.