[version.syn] Bump value of __cpp_lib_constexpr_complex (#6421)

P1383R2 "More constexpr for <cmath> and <complex>" modifies two headers;
both __cpp_lib_constexpr_cmath and __cpp_lib_constexpr_complex should be updated.

This aligns with existing practice in SD6.

source/support.tex

@@ -602,7 +602,7 @@
 #define @\defnlibxname{cpp_lib_constexpr_bitset}@                  202207L // also in \libheader{bitset}
 #define @\defnlibxname{cpp_lib_constexpr_charconv}@                202207L // also in \libheader{charconv}
 #define @\defnlibxname{cpp_lib_constexpr_cmath}@                   202306L // also in \libheader{cmath}, \libheader{cstdlib}
-#define @\defnlibxname{cpp_lib_constexpr_complex}@                 201711L // also in \libheader{complex}
+#define @\defnlibxname{cpp_lib_constexpr_complex}@                 202306L // also in \libheader{complex}
 #define @\defnlibxname{cpp_lib_constexpr_dynamic_alloc}@           201907L // also in \libheader{memory}
 #define @\defnlibxname{cpp_lib_constexpr_functional}@              201907L // freestanding, also in \libheader{functional}
 #define @\defnlibxname{cpp_lib_constexpr_iterator}@                201811L // freestanding, also in \libheader{iterator}