Fix test_daily_logger

* Updated bundled fmt to version 11.1.3

* Bump version to 1.15.1

include/spdlog/fmt/fmt.h

@@ -27,5 +27,4 @@
 #else  // SPDLOG_FMT_EXTERNAL is defined - use external fmtlib
     #include <fmt/core.h>
     #include <fmt/format.h>
-    #include <fmt/xchar.h>
 #endif

tests/test_daily_logger.cpp

@@ -46,12 +46,10 @@ TEST_CASE("daily_logger with dateonly calculator", "[daily_logger]") {
 
 struct custom_daily_file_name_calculator {
     static spdlog::filename_t calc_filename(const spdlog::filename_t &basename, const tm &now_tm) {
-        filename_memory_buf_t w;
-        spdlog::fmt_lib::format_to(std::back_inserter(w), SPDLOG_FILENAME_T("{}{:04d}{:02d}{:02d}"),
+        
+        return spdlog::fmt_lib::format(SPDLOG_FILENAME_T("{}{:04d}{:02d}{:02d}"),
                                    basename, now_tm.tm_year + 1900, now_tm.tm_mon + 1,
                                    now_tm.tm_mday);        
-
-        return SPDLOG_BUF_TO_STRING(w);
     }
 };