switch experimental::fs to std::fs

std::filesystem was added to C++17 and is well supported now.  Remove
the older std::experimental::filesystem usage.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I873667cf78efdf4f8ee2b63e8299783a5cba5ae4
diff --git a/settings_manager.mako.hpp b/settings_manager.mako.hpp
index 13bdc93..a1ee9ce 100644
--- a/settings_manager.mako.hpp
+++ b/settings_manager.mako.hpp
@@ -37,7 +37,7 @@
 #include <cereal/types/vector.hpp>
 #include <fstream>
 #include <utility>
-#include <experimental/filesystem>
+#include <filesystem>
 #include <regex>
 #include <phosphor-logging/elog.hpp>
 #include <phosphor-logging/elog-errors.hpp>
@@ -55,7 +55,7 @@
 namespace settings
 {
 
-namespace fs = std::experimental::filesystem;
+namespace fs = std::filesystem;
 
 namespace persistent
 {