Humans are severely skill issued ⚠️ Change HvN difficulty to Medium (#2971)

## Description:

For v29

HvN winrate is between 10 and 15%, but should be around 50%.

1. Change HvN difficulty to Medium
2. Little balance change in `NationAllianceBehavior`

## 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:

FloPinguin

src/core/execution/nation/NationAllianceBehavior.ts

@@ -240,13 +240,13 @@ export class NationAllianceBehavior {
     const { difficulty } = this.game.config().gameConfig();
     switch (difficulty) {
       case Difficulty.Easy:
-        return false; // 0% chance to reject on easy
+        return this.random.nextInt(0, 100) < 25; // 25% chance to reject on easy
       case Difficulty.Medium:
-        return this.random.nextInt(0, 100) < 20; // 20% chance to reject on medium
+        return this.random.nextInt(0, 100) < 50; // 50% chance to reject on medium
       case Difficulty.Hard:
-        return this.random.nextInt(0, 100) < 40; // 40% chance to reject on hard
+        return this.random.nextInt(0, 100) < 75; // 75% chance to reject on hard
       case Difficulty.Impossible:
-        return this.random.nextInt(0, 100) < 60; // 60% chance to reject on impossible
+        return true; // 100% chance to reject on impossible
       default:
         assertNever(difficulty);
     }

src/server/MapPlaylist.ts

@@ -128,7 +128,7 @@ export class MapPlaylist {
       publicGameModifiers: { isCompact, isRandomSpawn, startingGold },
       startingGold,
       difficulty:
-        playerTeams === HumansVsNations ? Difficulty.Hard : Difficulty.Easy,
+        playerTeams === HumansVsNations ? Difficulty.Medium : Difficulty.Easy,
       infiniteGold: false,
       infiniteTroops: false,
       maxTimerValue: undefined,