KDL 2.0.0 compliance

KDL 2.0.0 is about to be finally tagged (https://github.com/kdl-org/kdl/pull/434), and we're hoping the implementation community starts moving over to the new version now, which will be the "blessed" KDL going forward.

(I know this is already planned, but this is so I can put it in the [compliance tracking issue](https://github.com/kdl-org/kdl/issues/372).)