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/stack_spill.cpp

@@ -1,8 +1,10 @@
+#include "stack_spill.h"
+
 #include <stdexcept>
 
 #include "decompiler/Disasm/DecompilerLabel.h"
-#include "stack_spill.h"
-#include "third-party/fmt/core.h"
+
+#include "fmt/core.h"
 
 namespace decompiler {