Log, client/Response: adapt to libfmt 8.0.0 API changes

src/Log.cxx

@@ -35,7 +35,11 @@ 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()});
 }