chore!: drop support for Node 10

[ehmicky]

Part of #3741

This drops support for Node 10. This should not be a breaking change for sites (see explanation in #3741), but it requires making a major release.

This PR only drops the version in engines.node. Some of the code can be optimized thanks to dropping Node 10, but I will do this in a separate PR.

I will also fix the branch protection rule right before merging this.

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[JGAntunes]

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