PEL: switch fmt::format to use std::format

fmt::format is supported in the c++ std. This will
    help to remove fmt package dependency.

Change-Id: I89f0a5b67bbfe54168a20e93c989a1ae87f54503
Signed-off-by: Jayanth Othayoth <ojayanth@in.ibm.com>
diff --git a/extensions/openpower-pels/user_header.cpp b/extensions/openpower-pels/user_header.cpp
index c7fff1a..db61af8 100644
--- a/extensions/openpower-pels/user_header.cpp
+++ b/extensions/openpower-pels/user_header.cpp
@@ -20,10 +20,9 @@
 #include "pel_values.hpp"
 #include "severity.hpp"
 
-#include <fmt/format.h>
-
 #include <phosphor-logging/log.hpp>
 
+#include <format>
 #include <iostream>
 
 namespace openpower
@@ -71,7 +70,7 @@
         if (subsystemString == "invalid")
         {
             log<level::WARNING>(
-                fmt::format(
+                std::format(
                     "UH: Invalid SubSystem value in PEL_SUBSYSTEM: {:#X}",
                     eventSubsystem)
                     .c_str());
@@ -230,7 +229,7 @@
     catch (const std::exception& e)
     {
         log<level::ERR>(
-            fmt::format("Cannot unflatten user header: {}", e.what()).c_str());
+            std::format("Cannot unflatten user header: {}", e.what()).c_str());
         _valid = false;
     }
 }
@@ -241,7 +240,7 @@
     if (header().id != static_cast<uint16_t>(SectionID::userHeader))
     {
         log<level::ERR>(
-            fmt::format("Invalid user header section ID: {0:#x}", header().id)
+            std::format("Invalid user header section ID: {0:#x}", header().id)
                 .c_str());
         failed = true;
     }
@@ -249,7 +248,7 @@
     if (header().version != userHeaderVersion)
     {
         log<level::ERR>(
-            fmt::format("Invalid user header version: {0:#x}", header().version)
+            std::format("Invalid user header version: {0:#x}", header().version)
                 .c_str());
         failed = true;
     }