Migrate codebase to C++20

- VCMI can now be compiled in C++20 mode
- Replaced all references to C++17 with C++20
- Boost 1.74 is now set as minimal version (older version might work but
untested)
- Updated documentation to reflect required versions of compilers /
libraries
- Removed excessive fail-fast / continue-on-error from CI

client/mapView/MapRendererContext.h

@@ -13,10 +13,7 @@
 
 #include "../lib/GameConstants.h"
 #include "../lib/int3.h"
-
-VCMI_LIB_NAMESPACE_BEGIN
-struct ObjectPosInfo;
-VCMI_LIB_NAMESPACE_END
+#include "../lib/spells/ViewSpellInt.h"
 
 struct MapRendererContextState;