Seal the ruleset creation methods and let abstract method take their place

Also makes `CreatePlayerRuleset()` and `CreateRulesetForSkinProvider()` not-null to avoid unwanted behaviour with their derivers

osu.Game.Rulesets.Catch.Tests/TestSceneCatchPlayer.cs

@@ -16,6 +16,6 @@ namespace osu.Game.Rulesets.Catch.Tests
             typeof(CatchRuleset),
         };
 
-        protected override Ruleset CreateRuleset() => new CatchRuleset();
+        protected override Ruleset CreatePlayerRuleset() => new CatchRuleset();
     }
 }