[AIT-553] Update to ably-js 2.20.0 in examples and use new subscription fields

[VeskeR]

Resolves AIT-553

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• Commits have been rebased.
• Linting has been run against the changed file(s).
• The PR adheres to the writing style guide and contribution guide.

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[VeskeR]

@m-hulbert example page in the preview app https://ably-docs-ait-553-lo-ex-gxgi6q.herokuapp.com/examples/liveobjects-live-map?lang=javascript doesn't seem to use the latest ably-js version specified in this PR (2.20.0) - the protocol version used is still 5 in the preview app (should be 6). Did I do something wrong and need to update something else?