feat: remove top-level write concern options

[HanaPearlman]

Description

Code changes include: the WriteConcern type now has only one property: writeConcern: WriteConcern | WriteConcernSettings, where WriteConcernSettings (naming up for debate) encompasses the old top-level options. WriteConcern.fromOptions ignores the old top-level WC options.

The change to legalOptionNames in db.ts means the old top-level WC options will be filtered out when creating a Db. The change to validOptionNames in operations/connect.ts means it will either error or warn (configurable) if the old top-level WC options are passed to a MongoClient.

Test changes include everything from #2624 and now all options that are not being used to create a connection string use the writeConcern key.