Update Britannia and take its Classic version off rotation (#3710)

## Description:

Similar to how Europe classic was taken off rotation, we do the same
with Britannia. The newer Britannia was slightly updated to include the
rivers of the classic version, this is because of feedback from players
in the discord server who say the rivers in the classic version are an
important part of gameplay. Map otherwise keeps same nations and size.

## 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:

tri.star1011

resources/maps/britannia/manifest.json

@@ -1,17 +1,17 @@
 {
   "map": {
     "height": 2088,
-    "num_land_tiles": 1188359,
+    "num_land_tiles": 1183898,
     "width": 1600
   },
   "map16x": {
     "height": 522,
-    "num_land_tiles": 70697,
+    "num_land_tiles": 70124,
     "width": 400
   },
   "map4x": {
     "height": 1044,
-    "num_land_tiles": 292283,
+    "num_land_tiles": 290775,
     "width": 800
   },
   "name": "Britannia",

src/server/MapPlaylist.ts

@@ -37,7 +37,6 @@ const frequency: Partial<Record<GameMapName, number>> = {
   BetweenTwoSeas: 5,
   BlackSea: 6,
   Britannia: 5,
-  BritanniaClassic: 4,
   DeglaciatedAntarctica: 4,
   EastAsia: 5,
   Europe: 7,

tests/MapConsistency.test.ts

@@ -28,6 +28,7 @@ const FREQUENCY_EXEMPTIONS: Set<GameMapName> = new Set([
   "Tourney3",
   "Tourney4",
   "EuropeClassic",
+  "BritanniaClassic",
 ]);
 
 /** Parse the main.go maps registry and return the set of non-test map folder names. */