5.7.21-0ubuntu0.16.04.1) is not available, but a newer one is (5.7.25-0ubuntu0.16.04.2). We redirected you there.
/usr/share/doc/mysql-server/changelog.Debian.gz is in mysql-server 5.7.25-0ubuntu0.16.04.2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/mysql-server/../mysql-client-core-5.7/changelog.Debian.gz