fix: adapt to fmt 9.0.0 breaking changes

[alebastr]

Addresses the following breaking change:

Disabled automatic std::ostream insertion operator (operator<<) discovery when fmt/ostream.h is included to prevent ODR violations. You can get the old behavior by defining FMT_DEPRECATED_OSTREAM but this will be removed in the next major release. Use fmt::streamed or fmt::ostream_formatter to enable formatting via std::ostream instead.

The patch is good enough to fix builds in distributions updating to fmt 9. WIP because it's already late and I need to give it another look tomorrow; there's one more deprecation warning I want to address:

../subprojects/fmt/include/fmt/core.h:1711:43: warning: 'map<mpd_idle, 0>' is deprecated [-Wdeprecated-declarations]           
  const auto& arg = arg_mapper<Context>().map(std::forward<T>(val));                                                           
                                          ^                                                                                                                                                                                                                    
../subprojects/fmt/include/fmt/core.h:1451:3: note: 'map<mpd_idle, 0>' has been explicitly marked deprecated here
  FMT_DEPRECATED FMT_CONSTEXPR FMT_INLINE auto map(const T& val)                                                               
  ^                                                                                                                                                                                                                                                                                                                                                                                                                                       
...
../src/modules/mpd/state.cpp:81:11: note: in instantiation of function template specialization 'spdlog::debug<mpd_idle &>' requested here
  spdlog::debug("mpd: Idle: recv_idle events -> {}", events);

[Alexays]

Thanks @alebastr!