Fixup: bump feature-test macro

Author: eisenwave

source/support.tex

@@ -708,7 +708,7 @@
 #define @\defnlibxname{cpp_lib_freestanding_utility}@              202306L // freestanding, also in \libheader{utility}
 #define @\defnlibxname{cpp_lib_freestanding_variant}@              202311L // freestanding, also in \libheader{variant}
 #define @\defnlibxname{cpp_lib_fstream_native_handle}@             202306L // also in \libheader{fstream}
-#define @\defnlibxname{cpp_lib_function_ref}@                      202306L // also in \libheader{functional}
+#define @\defnlibxname{cpp_lib_function_ref}@                      202511L // also in \libheader{functional}
 #define @\defnlibxname{cpp_lib_gcd_lcm}@                           201606L // also in \libheader{numeric}
 #define @\defnlibxname{cpp_lib_generator}@                         202207L // also in \libheader{generator}
 #define @\defnlibxname{cpp_lib_generic_associative_lookup}@        201304L // also in \libheader{map}, \libheader{set}