Fix Java 8 compatibility in sequenced-collections old code example

[Copilot]

The oldCode block in sequenced-collections.yaml uses var (Java 10+) despite being labeled "Java 8".

• Replace var with Object in the old code example to match the declared Java 8 compatibility

# Before
oldCode: |-
  var last = list.get(list.size() - 1);
  var first = list.get(0);

# After
oldCode: |-
  Object last = list.get(list.size() - 1);
  Object first = list.get(0);