Bump NodeJS version requirement to v16 or higher

[akchinSTC]

Due to NodeJS 12 being already EOL and NodeJS 14 doing the same by this time next year, we should increase the minimum required version

What changes were proposed in this pull request?

How was this pull request tested?

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[elyra-bot]

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[ptitzler]

One more: https://github.com/elyra-ai/elyra/blob/master/docs/source/getting_started/installation.md#prerequisites

[ptitzler]

LGTM