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.

goalc/compiler/CodeGenerator.cpp

@@ -14,7 +14,7 @@
 #include "goalc/debugger/DebugInfo.h"
 #include "goalc/emitter/IGen.h"
 
-#include "third-party/fmt/core.h"
+#include "fmt/core.h"
 
 using namespace emitter;
 
@@ -318,4 +318,4 @@ void CodeGenerator::do_asm_function(FunctionEnv* env, int f_idx, bool allow_save
     // do the actual op
     ir->do_codegen(&m_gen, allocs, i_rec);
   }
-}
+}