scripted-diff: Bump copyright headers

-BEGIN VERIFY SCRIPT-
./contrib/devtools/copyright_header.py update ./
-END VERIFY SCRIPT-

src/clientversion.cpp

@@ -1,4 +1,4 @@
-// Copyright (c) 2012-2018 The Bitcoin Core developers
+// Copyright (c) 2012-2020 The Bitcoin Core developers
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.