feat(NODE-5504)!: bump bson major version

[durran]

Description

Bumps BSON_MAJOR_VERSION constant to 6.

What is changing?

Bumps BSON_MAJOR_VERSION constant to 6 and adds a test.

Is there new documentation needed for these changes?

None

What is the motivation for this change?

NODE-5504

Release Highlight

BSON_MAJOR_VERSION bumped to 6

Only BSON objects that have this major version can be serialized with this version of the library. Mismatched objects will throw a BSONVersionError when attempting to serialize.

Double check the following

• Ran npm run check:lint script
• Self-review completed using the steps outlined here
• PR title follows the correct format: type(NODE-xxxx)[!]: description
  • Example: feat(NODE-1234)!: rewriting everything in coffeescript
• Changes are covered by tests
• New TODOs have a related JIRA ticket

[W-A-James]

You're getting failures in CI because we have tests for what seems to be all of our BSON types that check for the @@mdb.bson.version flag that are failing to match the new value.