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.

common/util/print_float.cpp

@@ -5,8 +5,8 @@
 #include "common/goal_constants.h"
 #include "common/util/Assert.h"
 
+#include "fmt/core.h"
 #include "third-party/dragonbox.h"
-#include "third-party/fmt/core.h"
 
 /*!
  * Convert a float to a string. The string is _always_ in this format:
@@ -219,4 +219,4 @@ bool proper_float(float value) {
   } else {
     return true;
   }
-}
+}