feat(PlayerExecution): downgrade defense posts on capture (#1957)

## Description:

Closes https://github.com/openfrontio/OpenFrontIO/issues/1619.

On capture, defense posts will be downgraded.

On the live version this means defense posts will be destroyed, as
defense posts can only be level 1.

Misc. changes:
- added `decreaserLevel` helper
- cleaned up if/else in tick unit loop for clarity to avoid yet another
nested layer

Continuation of the stale PR,
https://github.com/openfrontio/OpenFrontIO/pull/1622.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

Discord username: `seekerreturns`

src/core/game/UnitImpl.ts

@@ -397,6 +397,18 @@ export class UnitImpl implements Unit {
     this.mg.addUpdate(this.toUpdate());
   }
 
+  decreaseLevel(destroyer?: Player): void {
+    this._level--;
+    if ([UnitType.MissileSilo, UnitType.SAMLauncher].includes(this.type())) {
+      this._missileTimerQueue.pop();
+    }
+    if (this._level <= 0) {
+      this.delete(true, destroyer);
+      return;
+    }
+    this.mg.addUpdate(this.toUpdate());
+  }
+
   trainType(): TrainType | undefined {
     return this._trainType;
   }