Remove u32

It's better to use types from the standard library.

source/graphics/SkeletonAnimDef.cpp

@@ -1,4 +1,4 @@
-/* Copyright (C) 2021 Wildfire Games.
+/* Copyright (C) 2026 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -31,7 +31,7 @@
 namespace
 {
 // Start IDs at 1 to leave 0 as a special value.
-u32 g_NextSkeletonDefUID = 1;
+std::uint32_t g_NextSkeletonDefUID = 1;
 }
 
 ///////////////////////////////////////////////////////////////////////////////////////////