Remove references to internal APIs

[achrinza]

MongoDB native driver v4.5.0 has marked existing APIs, of which Topology is utilised by this connector, as internal (PR: https://github.com/mongodb/node-mongodb-native/pull/2810, issue: https://jira.mongodb.org/browse/NODE-3245). It should be noted that these APIs have always intended to be internal, and that the added internal markings in v4.5.0 are a formality.

This connector references Topology APIs at these locations:

• https://github.com/loopbackio/loopback-connector-mongodb/blob/21a25fe8f42073c85784fe7e19f6272495a0fada/lib/mongodb.js#L527
• https://github.com/loopbackio/loopback-connector-mongodb/blob/21a25fe8f42073c85784fe7e19f6272495a0fada/lib/mongodb.js#L531

This issue is to track replacing references to these APIs.