Make db.setSecondaryOk()/mongo.setSecondaryOk()/rs.secondaryOk() deprecated aliases for setReadPref

As discussed on Slack, we should consider making

• db.setSecondaryOk()
• mongo.setSecondaryOk()
• rs.secondaryOk()

aliases for db.getMongo().setReadPref('primaryPreferred') if the current read preference is 'primary', otherwise no-op.

If not, the compatibility page at https://docs.mongodb.com/mongodb-shell/reference/compatibility/#deprecated-methods should be updated to mark these methods as removed rather than deprecated.