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:
- https://github.com/boostorg/signals2/commit/15fcf213563718d2378b6b83a1614680a4fa8cec
- https://github.com/boostorg/test/commit/495c095dc063052ce54f2fe9217fe0fc69ced5f1

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

doc/dependencies.md

@@ -17,7 +17,7 @@ You can find installation instructions in the `build-*.md` file for your platfor
 
 | Dependency | Releases | Version used | Minimum required | Runtime |
 | --- | --- | --- | --- | --- |
-| [Boost](../depends/packages/boost.mk) | [link](https://www.boost.org/users/download/) | [1.81.0](https://github.com/bitcoin/bitcoin/pull/26557) | [1.64.0](https://github.com/bitcoin/bitcoin/pull/22320) | No |
+| [Boost](../depends/packages/boost.mk) | [link](https://www.boost.org/users/download/) | [1.81.0](https://github.com/bitcoin/bitcoin/pull/26557) | [1.73.0](https://github.com/bitcoin/bitcoin/pull/29066) | No |
 | [libevent](../depends/packages/libevent.mk) | [link](https://github.com/libevent/libevent/releases) | [2.1.12-stable](https://github.com/bitcoin/bitcoin/pull/21991) | [2.1.8](https://github.com/bitcoin/bitcoin/pull/24681) | No |
 | glibc | [link](https://www.gnu.org/software/libc/) | N/A | [2.27](https://github.com/bitcoin/bitcoin/pull/27029) | Yes |
 | Linux Kernel | [link](https://www.kernel.org/) | N/A | [3.17.0](https://github.com/bitcoin/bitcoin/pull/27699) | Yes |