AthenaKernel: C++20 fixes for DsoDb

Fix build failure with C++20.

I am fixing this deliberately also in 23.0 since that's where the bug got introduced by !64255 (merged).

Control/AthenaKernel/src/DsoDb.cxx

@@ -74,7 +74,7 @@ namespace {
   inline
   std::string to_string(const fs::path& p) 
   {
-    return p.u8string();
+    return p.string();
   }
 
   std::string getlibname(const std::string& libname)