lib/fmt: require libfmt 9

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

src/io/BufferedOutputStream.cxx

@@ -52,11 +52,7 @@ BufferedOutputStream::VFmt(fmt::string_view format_str, fmt::format_args args)
 	/* TODO format into this object's buffer instead of allocating
 	   a new one */
 	fmt::memory_buffer b;
-#if FMT_VERSION >= 80000
 	fmt::vformat_to(std::back_inserter(b), format_str, args);
-#else
-	fmt::vformat_to(b, format_str, args);
-#endif
 	return Write(std::as_bytes(std::span{b.data(), b.size()}));
 }
 

src/io/BufferedOutputStream.hxx

@@ -7,9 +7,6 @@
 #include "util/SpanCast.hxx"
 
 #include <fmt/core.h>
-#if FMT_VERSION >= 80000 && FMT_VERSION < 90000
-#include <fmt/format.h>
-#endif
 
 #include <cstddef>
 #include <string_view>
@@ -71,14 +68,8 @@ public:
 
 	template<typename S, typename... Args>
 	void Fmt(const S &format_str, Args&&... args) {
-#if FMT_VERSION >= 90000
 		VFmt(format_str,
 		     fmt::make_format_args(args...));
-#else
-		VFmt(fmt::to_string_view(format_str),
-		     fmt::make_args_checked<Args...>(format_str,
-						     args...));
-#endif
 	}
 
 #ifdef _UNICODE