Irritatingly, MySQL 8 reserves (but does not use) the keyword "groups," so if you happened to have something like this:
SELECT name FROM groups where user=5;
Then as soon as you upgraded to MySQL 8, your query would be a syntax error.
This will work, though:
SELECT name FROM `groups` where user=5;
Naturally, Bugzilla has a table named "groups", so this broke.
Ubuntu started shipping MySQL 8 in 2019, and there was never a Bugzilla 5.0.7 release to fix this (the unreleased 5.2 branch supposedly has this fixed, but I'm not sure).
If you, like me, just updated your old Ubuntu server and found--surprise!--that the stable Bugzilla release no longer works, here's a patch to 5.0.6 to fix it.
I can't say this is correct, complete, or sane, but it got the bug tracker up and running again here.