Merge #20480: Replace boost::variant with std::variant

faa8f68943 Replace boost::variant with std::variant (MarcoFalke)

Pull request description:

  Now that we can use std::variant from the vanilla standard library, drop the third-party boost variant dependency

ACKs for top commit:
  fjahr:
    Code review ACK faa8f68943
  fanquake:
    ACK faa8f68943

Tree-SHA512: 6e3aecd33b00c2e31a763f999247944d5b2ce5e3018f1965c516c1000cd08ff6703a8d50fb0be64883153da2925ae72986b8a6b96586db74057bd05d6f4986e6

src/util/message.cpp

@@ -31,7 +31,7 @@ MessageVerificationResult MessageVerify(
         return MessageVerificationResult::ERR_INVALID_ADDRESS;
     }
 
-    if (boost::get<PKHash>(&destination) == nullptr) {
+    if (std::get_if<PKHash>(&destination) == nullptr) {
         return MessageVerificationResult::ERR_ADDRESS_NO_KEY;
     }