Use "node:" prefixed imports everywhere for node imports.

[boenrobot]

Adjust minimum node version and update CI accordingly.

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Using "node:" prefixed imports enables the use of this library in runtimes other than node where node compatibility layer is provided for prefixed imports (e.g. workerd). The cost is that the minimum nodejs version needs to be bumped to ">=14.18.0 <15 || >=16", where this feature was introduced.

[phated]

Node 10 won't be dropped until gulp v6 work begins, and we have a lot of work on v5 that still needs to be done.

I'll keep this open until that work begins.