Generated builder code should use List.copyOf(l) instead of Collections.unmodifiableList(new ArrayList<>(l))

[ljnelson]

Some generated builder code features:

Collections.unmodifiableList(new ArrayList<>(listToCopy));

…but should use:

List.copyOf(listToCopy);

…instead.

[tomas-langer]

Fixed by #7008

Still using other methods for Set and Map, as we want to preserve order of insertion.