Update `DefaultIssDetector` to skip PCES sigs at upgrade exactly

When nodes are gossiping signature txs during `FREEZING`, it becomes very likely that these signatures will become post-signature txs **after the upgrade's `onStateInitialized()` has changed the roster, and hash, of the freeze round's state**.

But without,
```
event.preconsensus.forceIgnorePcesSignatures=true
```
the `DefaultIssDetector` behavior on `main` [will be](https://github.com/hiero-ledger/hiero-consensus-node/blob/main/platform-sdk/swirlds-platform-core/src/main/java/com/swirlds/platform/state/iss/DefaultIssDetector.java#L360) will be to validate these signatures over the pre-upgrade state hash; and all nodes will mostly report an ISS shortly after upgrade.

So although we don't want to disable PCES signatures in general, we *do* want to do so an upgrade boundary.