How (and where) to configure mysql on Ubuntu 20.04

I am migrating a LAMP system from a Ubuntu 14.04 droplet to a Ubuntu 20.04 droplet. Most of my problems seem to be involved with moving from mysql 5.5.60 to 8.0.25. I’ve identified two main problems so far:

• I need to remove NO _ ZEROES _ IN _ DATE from the SQL mode, but can’t find where, in Digital Ocean’s LAMP setup, the mysql configuration is stored.
• It appears that mysql 8.0 does not have default default [sic] values for datatypes. Is there a way to define them? … to repair tables without them? … or to relax the requirement for default values, e.g. to simply use null where there is no default value?

These two problems interact in absurd ways. For example, an attempt to alter a column to add a default value fails with an error complaining that existing records contain zeroes in a date:

mysql> alter table meetings modify column meetMpgmKey int default 0;
ERROR 1292 (22007): Incorrect date value: '2004-05-00' for column 'meetBeginDate' at row 50

More generally, is there a guide to upgrading to mysql 8 that covers issues like this?