Update Agenda cleanup script to latest MongoDB client

[simison]

Run NODE_ENV=production npm run agenda-maintenance

Output before:

(node:920) [MONGODB DRIVER] Warning: Current Server Discovery and Monitoring engine is deprecated, and will be removed in a future version. To use the new Server Discover and Monitoring engine, pass option { useUnifiedTopology: true } to the MongoClient constructor.
(Use `node --trace-warnings ...` to show where the warning was created)
Connected to MongoDB: mongodb://localhost/trust-roots
/trustroots/node_modules/mongodb/lib/topologies/server.js:294
          throw err;
          ^

TypeError: dbConnection.collection is not a function
    at /trustroots/bin/db-maintenance/archive-done-agenda-jobs.js:117:42
    at /trustroots/node_modules/mongodb/lib/utils.js:704:5
    at /trustroots/node_modules/mongodb/lib/mongo_client.js:261:7
    at connectCallback (/trustroots/node_modules/mongodb/lib/operations/connect.js:365:5)
    at /trustroots/node_modules/mongodb/lib/operations/connect.js:458:5
    at Server.connectHandler (/trustroots/node_modules/mongodb/lib/topologies/server.js:291:9)
    at Object.onceWrapper (events.js:483:26)
    at Server.emit (events.js:376:20)
    at Pool.<anonymous> (/trustroots/node_modules/mongodb/lib/core/topologies/server.js:384:12)
    at Pool.emit (events.js:376:20)

Output after:

(node:1983) [MONGODB DRIVER] Warning: Current Server Discovery and Monitoring engine is deprecated, and will be removed in a future version. To use the new Server Discover and Monitoring engine, pass option { useUnifiedTopology: true } to the MongoClient constructor.
(Use `node --trace-warnings ...` to show where the warning was created)
Connected to MongoDB: mongodb://localhost/trust-roots
Counting docs...
Going to move 891087 documents from agendaJobs to agendaJobsArchived


Source count: 891096
Target count: 125740
Total: 1016836

Fetching docs for transfer...


Processing 891087 docs...