ENH: adjust fileName methods for similarity to std::filesystem::path

- stem(), replace_name(), replace_ext(), remove_ext() etc

- string::contains() method - similar to C++23 method

  Eg,
      if (keyword.contains('/')) ...
  vs
      if (keyword.find('/') != std::string::npos) ...

src/functionObjects/field/zeroGradient/zeroGradient.H

@@ -137,10 +137,6 @@ class zeroGradient
 
     // Private Member Functions
 
-        //- Check that string contains the appropriate substitution token(s)
-        static bool checkFormatName(const std::string& str);
-
-
         //- Accept unless field only has constraint patches
         //  (ie, empty/zero-gradient/processor)
         //  This should also avoid fields that were already processed by