Upgrade minimum node support on analytics-node to 14.x

[silesky]

Node 12 LTS has been at complete end-of-life for 6 months, we should not include any official support for it. This change also allows us to compile to ES2020 (smaller package and allows us to use actual JS private fields / methods if we want).

We already use node 14 for buildkite runner, so no change needed.

[changeset-bot]

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Latest commit: d757a70

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