Opencast removed/changed some classes from `org.opencastproject.util` that we depend on

[JulianKniephoff]

That makes the tool currently no longer build with Opencast 19 upwards. You can still build against 18, but that will only defer the problem to runtime.

[JulianKniephoff]

Note that #659 fixes this only temporarily. In the long run, we want to rip out these classes!