Comment 2 for bug 1849694

Hi Brian,
from your log:
mysqld will log errors to /var/log/mysql/error.log
2019-10-24T14:30:52.608720Z 0 [ERROR] [MY-010946] [Server] Failed to start mysqld daemon. Check mysqld error log.
Warning: Unable to start the server.
Job for mysql.service failed because the control process exited with error code.
See "systemctl status mysql.service" and "journalctl -xe" for details.
invoke-rc.d: initscript mysql, action "start" failed.

And if we check said error log:
2019-10-24T15:17:00.139004Z 0 [ERROR] [MY-010119] [Server] Aborting
2019-10-24T15:17:01.204165Z 0 [System] [MY-010910] [Server] /usr/sbin/mysqld: Shutdown complete (mysqld 8.0.17-0ubuntu2) (Ubuntu).
2019-10-24T15:17:01.978668Z 0 [Warning] [MY-011068] [Server] The syntax 'expire-logs-days' is deprecated and will be removed in a future release. Please use binlog_expire_logs_seconds instead.
2019-10-24T15:17:01.981218Z 0 [System] [MY-010116] [Server] /usr/sbin/mysqld (mysqld 8.0.17-0ubuntu2) starting as process 3984
2019-10-24T15:17:04.278931Z 0 [Warning] [MY-010068] [Server] CA certificate ca.pem is self signed.
2019-10-24T15:17:04.285654Z 0 [ERROR] [MY-000067] [Server] unknown variable 'query_cache_limit=1M'.

Now that points to 'query_cache_limit=1M' being in your config but not (or no more) being supported.
And [1] confirms this.
Default configurations are upgraded, but yours seems to be customized, so you'd need to adapt your local configuration to be able to work with the newer version of mysql - then you will be able to restart the service and that will also allow the upgrade to proceed.

[1]: https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_query_cache_limit