libconsensus: deprecate

This library has existed for nearly 10 years with very little known uptake or
impact. It has become a maintenance burden. In several cases it dictates our
code/library structure (for example necessitating LIBBITCOIN_CRYPTO_BASE), as
well as build-system procedures (building multiple copies of object files
especially for the lib).

Several discussions have arisen wrt migrating it to CMake and it has become
difficult to justify adding more complexity for a library that is virtually
unused anyway.

See for example the discussions:
hebasto#41
bitcoin#29123

Instead, we (fanquake, hebasto, TheCharlatan, and I) propose simply not
migrating it to CMake and letting it end with v26. Any remaining use-cases
could be handled in the future by libbitcoinkernel.

doc/shared-libraries.md

@@ -2,6 +2,7 @@ Shared Libraries
 ================
 
 ## bitcoinconsensus
+***This library is deprecated and will be removed in a future release***
 
 The purpose of this library is to make the verification functionality that is critical to Bitcoin's consensus available to other applications, e.g. to language bindings.