lib/fmt: drop use of FMT_STRING

When compiling with libfmt-11.1.0 and newer the following compile errors occur:

In file included from ../src/decoder/DecoderPrint.cxx:23:
../src/client/Response.hxx: In instantiation of 'bool Response::Fmt(const S&, Args&& ...) [with S = decoder_plugin_print(Response&, const DecoderPlugin&)::<lambda()>::FMT_COMPILE_STRING; Args = {const char* const&}]':
../src/decoder/DecoderPrint.cxx:38:7:   required from here
   38 |         r.Fmt(FMT_STRING("plugin: {}\n"), plugin.name);
      |         ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/client/Response.hxx:86:28: error: cannot convert 'const decoder_plugin_print(Response&, const DecoderPlugin&)::<lambda()>::FMT_COMPILE_STRING' to 'fmt::v11::string_view' {aka 'fmt::v11::basic_string_view<char>'}
   86 |                 return VFmt(format_str,
      |                        ~~~~^~~~~~~~~~~~
   87 |                             fmt::make_format_args(args...));
      |                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/client/Response.hxx:81:36: note: initializing argument 1 of 'bool Response::VFmt(fmt::v11::string_view, fmt::v11::format_args)'
   81 |         bool VFmt(fmt::string_view format_str, fmt::format_args args) noexcept;
      |                   ~~~~~~~~~~~~~~~~~^~~~~~~~~~
../src/client/Response.hxx: In instantiation of 'bool Response::Fmt(const S&, Args&& ...) [with S = decoder_plugin_print(Response&, const DecoderPlugin&)::<lambda()>::FMT_COMPILE_STRING; Args = {const char* const&}]':

The error is due to the use of FMT_STRING. The libfmt team shared the following:

    The correct way of using FMT_STRING is to wrap a format string when passing to a
    function with compile-time checks (i.e. that takes format_string) as documented
    in https://fmt.dev/11.1/api/#legacy-compile-time-checks.

    Noting that FMT_STRING is a legacy API and has been superseded by consteval-based
    API starting from version 8: https://github.com/fmtlib/fmt/releases/tag/8.0.0. It
    looks like MPD is trying to emulate {fmt}'s old way of implementing compile-time
    checks which was never properly documented because it was basically a hack. So the
    correct fix is to switch to format_string and, possibly, remove usage of FMT_STRING.

    The old way of doing compile-time checks (fmt::make_args_checked) was documented
    in https://fmt.dev/7.1/api.html#argument-lists but it looks like MPD is not using
    that API so the problematic uses of FMT_STRING have no effect and can just be removed.

The FMT_STRING has been removed in this change based on the fmt-7.1 API and now MPD is
successfully compile against the current libfmt-11.1.0 which highlighted the issue that
had been present in the codebase as it is now triggering the error, is legacy and was
not using the API for which FMT_STRING was aligned with.

src/command/FileCommands.cxx

@@ -85,12 +85,12 @@ handle_listfiles_local(Response &r, Path path_fs)
 			continue;
 
 		if (fi.IsRegular())
-			r.Fmt(FMT_STRING("file: {}\n"
-					 "size: {}\n"),
+			r.Fmt("file: {}\n"
+			      "size: {}\n",
 			      name_utf8,
 			      fi.GetSize());
 		else if (fi.IsDirectory())
-			r.Fmt(FMT_STRING("directory: {}\n"), name_utf8);
+			r.Fmt("directory: {}\n", name_utf8);
 		else
 			continue;
 
@@ -129,7 +129,7 @@ public:
 	void OnPair(StringView _key, StringView _value) noexcept override {
 		const std::string_view key{_key}, value{_value};
 		if (IsValidName(key) && IsValidValue(value))
-			response.Fmt(FMT_STRING("{}: {}\n"), key, value);
+			response.Fmt("{}: {}\n", key, value);
 	}
 };
 
@@ -218,7 +218,7 @@ read_stream_art(Response &r, const std::string_view art_directory,
 		read_size = is->Read(lock, buffer.get(), buffer_size);
 	}
 
-	r.Fmt(FMT_STRING("size: {}\n"), art_file_size);
+	r.Fmt("size: {}\n", art_file_size);
 
 	r.WriteBinary({buffer.get(), read_size});
 
@@ -348,10 +348,10 @@ public:
 			return;
 		}
 
-		response.Fmt(FMT_STRING("size: {}\n"), buffer.size);
+		response.Fmt("size: {}\n", buffer.size);
 
 		if (mime_type != nullptr)
-			response.Fmt(FMT_STRING("type: {}\n"), mime_type);
+			response.Fmt("type: {}\n", mime_type);
 
 		buffer.size -= offset;