2024-July-25 - 2.7.14823

Fix for crash during air battle (#12772)

Problem: 'oldUnits' is stateful and may already contain units.
We then "double add" a unit in the following code block

(RemoveUnitsHistoryChange.java:71)

```
    final Collection<Unit> allUnloadedUnits = new HashSet<>();
    for (Unit u : killedUnits) {
      oldUnits.add(u);
```

To resolve this issue, the 'oldUnits' and 'killedUnits' is converted
into a set, which removes duplicate units. (This 'fix' is not very deep,
arguably this is a hack. Why is the duplicate unit being added in the first
place?)

Resolves: #12770