These methods aren't implemented in `Set`, and don't really make sense because `Set` types can't be accessed by index.
Note: although this removes methods, this actually isn't a breaking change:
1. Calling `Set.findIndex()` currently throws an error around "cannot invoke abstract method"
2. `List` and `Set` are the only subclasses of `Collection`.
The following code isn't breaking at runtime, although static analysis tooling (like our IDE plugins) will now flag this as an error:
```pkl
myCollection: Collection
idx = myCollection.indexOf(1)
```