ci: Upgrade IWYU to 0.26 compatible with Clang 22

Additionally, removes mapping workarounds for issues that have been
fixed upstream.

src/kernel/coinstats.cpp

@@ -20,11 +20,11 @@
 #include <util/overflow.h>
 #include <validation.h>
 
+#include <cstddef>
 #include <map>
 #include <memory>
 #include <span>
 #include <utility>
-#include <version>
 
 namespace kernel {