2023-October-06 - 2.6.14530

Fix transportedBy not cleared after a convoy capture. (#12017)

This fixes #12013, which was caused by units still having a stale transportedBy property set under some circumstances. This would happen when there's an uncontested convoy zone capture before a landing and where the transport didn't participate in the convoy zone capture.
This is a pre-2.6 bug.

Note: The issue resulting from this bug with air transports will only be fixed for new games, as existing save games will have the bad state already carried over from past turns.

Includes a unit test that reproduces the scenario.