lib/fmt: require libfmt 9

Version 9 added the "debug format" which I'd like to use.

src/Log.cxx

@@ -14,11 +14,7 @@ LogVFmt(LogLevel level, const Domain &domain,
 	fmt::string_view format_str, fmt::format_args args) noexcept
 {
 	fmt::memory_buffer buffer;
-#if FMT_VERSION >= 80000
 	fmt::vformat_to(std::back_inserter(buffer), format_str, args);
-#else
-	fmt::vformat_to(buffer, format_str, args);
-#endif
 	Log(level, domain, {buffer.data(), buffer.size()});
 }