deps: update fmt to latest version (#3403)

This updates `fmt` to the latest version and moves to just being a copy
of their repo to make updating easier (no editing their cmake / figuring
out which files to minimally include).

The motivation for this is now that we switched to C++ 20, there were a
ton of deprecated function usages that is going away in future compiler
versions. This gets rid of all those warnings.

decompiler/analysis/insert_lets.cpp

@@ -1,3 +1,5 @@
+#include "insert_lets.h"
+
 #include <algorithm>
 #include <array>
 #include <limits>
@@ -7,10 +9,10 @@
 #include "common/log/log.h"
 #include "common/util/Assert.h"
 #include "common/util/print_float.h"
+
 #include "decompiler/IR2/GenericElementMatcher.h"
 #include "decompiler/IR2/bitfields.h"
 #include "decompiler/util/DecompilerTypeSystem.h"
-#include "insert_lets.h"
 
 namespace decompiler {