settings: Change max value of skill level from 20 to 30

The reason to raise this maximum is because for the following game.

 1.     f4    d2
 2.     b4    d6
 3.     d1    d5
 4.     d7    c4
 5.     g1    a1
 6.     g4    e4
 7.     g7xe4    e4
 8.     a7xe4    e4
 9.     d3    a4
10.    d3-c3    d5-e5
11.    c3-d3    e5-d5
12.    d3-e3    c4-c5
13.    f4-f2    e4-f4
14.    b4-c4    d5-e5
15.    e3-e4    c5-d5
16.    c4-c5    d2-b2
17.    d1-d2    a1-d1
18.    d2-d3    b2-b4 (bad move 39. Should be a4-b4)

Set skill level to 24 and set thinking time to 0 (means infinite) can do the right move.

src/mills.cpp

@@ -628,7 +628,7 @@ Depth get_search_depth(const Position *pos)
     }
 
     // Do not too weak
-    if (depthLimit == 20 && d <= 4) {   // TODO
+    if (depthLimit == 30 && d <= 4) {   // TODO
         d = 4;
     }
 #endif

src/ucioption.cpp

@@ -180,7 +180,7 @@ void init(OptionsMap &o)
     o["Clear Hash"] << Option(on_clear_hash);
     o["Ponder"] << Option(false);
     o["MultiPV"] << Option(1, 1, 500);
-    o["SkillLevel"] << Option(1, 0, 20, on_skill_level);
+    o["SkillLevel"] << Option(1, 0, 30, on_skill_level);
     o["MoveTime"] << Option(1, 0, 60, on_move_time);
     o["AiIsLazy"] << Option(false, on_aiIsLazy);
     o["Move Overhead"] << Option(10, 0, 5000);

src/ui/flutter_app/lib/widgets/game_page.dart

@@ -1282,7 +1282,7 @@ class _GamePageState extends State<GamePage>
       return;
     }
 
-    bool isTopLevel = (Config.skillLevel == 20); // TODO: 20
+    bool isTopLevel = (Config.skillLevel == 30); // TODO: 30
 
     if (result == GameResult.win &&
         !isTopLevel &&

src/ui/flutter_app/lib/widgets/game_settings_page.dart

@@ -69,8 +69,8 @@ class _GameSettingsPageState extends State<GameSettingsPage> {
         child: Slider(
           value: Config.skillLevel.toDouble(),
           min: 1,
-          max: 20,
-          divisions: 19,
+          max: 30,
+          divisions: 29,
           label: Config.skillLevel.round().toString(),
           onChanged: (value) {
             setState(() {