Improve use of generics in DisjunctionDISIApproximation construction

[ChrisHegarty]

This is a small usability improvement which avoids the need to copy the DisiWrappers collection during the construction of a DisjunctionDISIApproximation just to satisfy generic constraints. Follows PECS.

Allows usage similar to:

List<MyDisiWrapper> wrappers = new ArrayList<>();
for (Scorer scorer : ..) {
  wrappers.add(new MyDisiWrapper(scorer, ..));
}
this.approximation = new DisjunctionDISIApproximation(wrappers, Long.MAX_VALUE);

relates #14052