[version.syn] Revert __cpp_lib_chrono back to 201907L as directed by @…

…brevzin in #3476.

source/support.tex

@@ -574,7 +574,7 @@
 #define @\defnlibxname{cpp_lib_byte}@                              201603L // also in \libheader{cstddef}
 #define @\defnlibxname{cpp_lib_char8_t}@                           201907L
   // also in \libheader{atomic}, \libheader{filesystem}, \libheader{istream}, \libheader{limits}, \libheader{locale}, \libheader{ostream}, \libheader{string}, \libheader{string_view}
-#define @\defnlibxname{cpp_lib_chrono}@                            201803L // also in \libheader{chrono}
+#define @\defnlibxname{cpp_lib_chrono}@                            201907L // also in \libheader{chrono}
 #define @\defnlibxname{cpp_lib_chrono_udls}@                       201304L // also in \libheader{chrono}
 #define @\defnlibxname{cpp_lib_clamp}@                             201603L // also in \libheader{algorithm}
 #define @\defnlibxname{cpp_lib_complex_udls}@                      201309L // also in \libheader{complex}