package:mysql-server-5.7:5.7.21-0ubuntu0.16.04.1
Installing new version of config file /etc/mysql/debian-start ...
Renaming removed key_buffer and myisam-recover options (if present)
ERROR: Unable to start MySQL server:
2018-04-19T01:33:21.748825Z 0 [Warning] option 'max_connections': unsigned value 1000000 adjusted to 100000
2018-04-19T01:33:21.889320Z 0 [ERROR] unknown variable 'log_slow_queries=/var/log/mysql/error.log'
2018-04-19T01:33:21.898062Z 0 [ERROR] Aborting
Please take a look at https://wiki.debian.org/Teams/MySQL/FAQ for tips on fixing common upgrade issues.
Once the problem is resolved, run apt-get --fix-broken install to retry.
dpkg: error processing package mysql-server-5.7 (--configure):
 subprocess installed post-installation script returned error exit status 1