build: Bump minimum required Boost to 1.73.0 to support C++20

Boost versions <1.73 have C++20-specific bugs that were fixed in the
following commits:
- 15fcf21356
- 495c095dc0

configure.ac

@@ -1454,7 +1454,7 @@ fi
 if test "$use_boost" = "yes"; then
 
   dnl Check for Boost headers
-  AX_BOOST_BASE([1.64.0],[],[AC_MSG_ERROR([Boost is not available!])])
+  AX_BOOST_BASE([1.73.0],[],[AC_MSG_ERROR([Boost is not available!])])
   if test "$want_boost" = "no"; then
     AC_MSG_ERROR([only libbitcoinconsensus can be built without Boost])
   fi