Drop Node 16 in ember-source 5.4

[kategengler]

Node 16 was only supported through early September. A new app on the 5.4 beta already does not work on Node 16 due to transitive dependencies that have updated engines.

[NullVoxPopuli]

(iirc) if we can get ember to a v2 addon, the node version will stop mattering for consumers, and in development, we can use "current" instead of "oldest supported LTS"

[kategengler]

Ok, but for now this is something that needs to be done in the next couple of weeks