Java: Deprecate and remove Element::getDoc

`Element::getDoc` only performs an implicit cast of `this`:
https://github.com/github/codeql/blob/f9973d10aea9c550d91f6607b7cfdd2f543d64ee/java/ql/src/semmle/code/java/Element.qll#L42-L43

- This is probably redundant because you can instead write a cast `element.(Document) ...`
- It is irritating because it suggests that every `Element` can be 'converted' to `Document`, which is not the case
- It causes irritating error messages when used on types which cannot be cast to `Document`:

Therefore it might be good to deprecate and remove the predicate.

Edit: `Package` is not a good example for an `Element` which does not extend `Document` since it should support documentation, see #5288. Though there are other `Element` subclasses such as `Import` or `Modifier` which cannot be documented in Java.

Relates to #3705