maint: Update copyright notices in --version output. List the most recent year in which changes were made, per <https://www.gnu.org/prep/standards/html_node/_002d_002dversion.html>. * build-aux/declared.sh (func_version): Say 2019. * build-aux/libtool-next-version (func_version): Likewise. * build-aux/run-test (func_version): Likewise. * check-module (COPYRIGHT_NOTICE): Say 2012. * lib/git-merge-changelog.c (main): Say 2020.