Codechange: Replace old non-standard attributes with C++17/20 standard attributes.

src/terraform_gui.cpp

@@ -83,7 +83,7 @@ static void GenerateRockyArea(TileIndex end, TileIndex start)
 		switch (GetTileType(tile)) {
 			case MP_TREES:
 				if (GetTreeGround(tile) == TREE_GROUND_SHORE) continue;
-				FALLTHROUGH;
+				[[fallthrough]];
 
 			case MP_CLEAR:
 				MakeClear(tile, CLEAR_ROCKS, 3);