Remove Node 12 and 15 support

[SebC99]

New Feature / Enhancement Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.

Current Limitation

Current build settings allow for node.js from version 8. But as far as I can see, it can only be used via parse-server which only supports nodejs from version 12.

Feature / Enhancement Description

Could we change node support for Parse SDK JS as well? It would allow using ES7 features as object destructuring without the need for a polyfill.

Example Use Case

As an example, these lines seems to be the slowest (by far) in Parse.Object getter, which is called a lot in any code. Replacing these lines with a simple const data = {...serverData}; is more than 80% faster according to jsbench.

[parse-github-assistant]

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[mtrezza]

I'd suggest:

1. Create a PR which sets the min node version for the JS SDK to >=12 and refactors the bottlenecks you mentioned
2. Publish a minor release in Parse Server with the new JS SDK

Later, when we are bumping the node version to >=14 in Parse Server, we can also so that in the JS SDK. The bump to Node 14 in Parse Server will happen in 4 months with Parse Sever 6.x, at the end of the year.

[mtrezza]

A PR for this should cover:

• Remove support for Node 12 and 15
• Add engines.node to package.json, not sure why this is currently missing
• Update Node compatibility table in README
• Update Node badge table in README