Upgrade to xxhash 0.8.2 as a Conan requirement, enable SIMD hashing (#4893)

We are currently using old version 0.6.2 of `xxhash`, as a verbatim copy and paste of its header file `xxhash.h`. Switch to the more recent version 0.8.2. Since this version is in Conan Center (and properly protects its ABI by keeping the state object incomplete), add it as a Conan requirement. Switch to the SIMD instructions (in the new `XXH3` family) supported by the new version.

Builds/CMake/RippledCore.cmake

@@ -26,7 +26,6 @@ target_sources (xrpl_core PRIVATE
   src/ripple/beast/clock/basic_seconds_clock.cpp
   src/ripple/beast/core/CurrentThreadName.cpp
   src/ripple/beast/core/SemanticVersion.cpp
-  src/ripple/beast/hash/impl/xxhash.cpp
   src/ripple/beast/insight/impl/Collector.cpp
   src/ripple/beast/insight/impl/Groups.cpp
   src/ripple/beast/insight/impl/Hook.cpp
@@ -153,7 +152,8 @@ target_link_libraries (xrpl_core
     secp256k1::secp256k1
     ed25519::ed25519
     date::date
-    Ripple::opts)
+    Ripple::opts
+    xxHash::xxhash)
 #[=================================[
    main/core headers installation
 #]=================================]
@@ -362,9 +362,6 @@ install (
     src/ripple/beast/hash/uhash.h
     src/ripple/beast/hash/xxhasher.h
   DESTINATION include/ripple/beast/hash)
-install (
-  FILES src/ripple/beast/hash/impl/xxhash.h
-  DESTINATION include/ripple/beast/hash/impl)
 install (
   FILES
   src/ripple/beast/net/IPAddress.h