[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