Rebalance nation difficulty 📊 More oriented towards beginners now (#3184)

## Description:

**3 problems:**

* Nation difficulty steps in `DefaultConfig` don't look good (max
troops: 0.5 → 1 → 1.125 → 1.25).
* We previously reduced the difficulty of easy nations for singleplayer,
but now they are too easy for public FFAs.
* In Discord we discussed HvN difficulty and concluded that a 50% human
win rate (my previous target) is too low; nations should be easier.

This PR addresses all of them:

* Difficulty steps in `DefaultConfig` are now cleaner (max troops: 0.5 →
0.75 → 1 → 1.25).
* Nation difficulty in public-game FFAs is restored to *medium* (0.75
max troops - still weaker than humans, but not too weak).
* HvN difficulty (medium) is now easier.

Regarding singleplayer:
These rebalances make nation difficulties more beginner-friendly, while
experts still have their challenge at *Impossible*.


## 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/server/MapPlaylist.ts

@@ -150,8 +150,7 @@ export class MapPlaylist {
         startingGold,
       },
       startingGold,
-      difficulty:
-        playerTeams === HumansVsNations ? Difficulty.Medium : Difficulty.Easy,
+      difficulty: Difficulty.Medium,
       infiniteGold: false,
       infiniteTroops: false,
       maxTimerValue: undefined,