Replace std::snprintf() with wpi::format_to_n_c_str() (#5645)

fmtlib uses consteval format string processing, which makes it more
efficient than std::snprintf().

snprintf()s in libuv, mpack, processstarter, and wpigui were left alone.
processstarter uses stdlib only, and wpigui only depends on imgui.

fmt::format_to_n() is analogous to std::format_to_n()
(https://en.cppreference.com/w/cpp/utility/format/format_to_n)

wpi::format_to_n_c_str() is a wrapper which adds the trailing NUL.

wpilibc/src/main/native/cpp/smartdashboard/Mechanism2d.cpp

@@ -4,7 +4,6 @@
 
 #include "frc/smartdashboard/Mechanism2d.h"
 
-#include <cstdio>
 #include <string_view>
 
 #include <networktables/NTSendableBuilder.h>