[keyserver] Avoid passing dbType to mysql2 in getMigrationConnection

Summary:
Otherwise we get this error printed:

Ignoring invalid configuration option passed to Connection: dbType. This is currently a warning, but in future versions of MySQL2, an error will be thrown if you pass an invalid configuration option to a Connection

See also D4529 and this update of D4516.

Depends on D4675

Test Plan: Flow

Reviewers: jacek, tomek, atul

Reviewed By: tomek

Subscribers: adrian, abosh

Differential Revision: https://phab.comm.dev/D4676