[Maintenance] drop support for node versions that are no longer security supported.

[emilyashley]

Which problem is this PR solving?

• closes #296
• unblocks dependency updates #293 #304

Short description of the changes

• CircleCI to only test Node 14, 16, 18 (current security supported node versions)
• Update readme and package.json and package lock to Node >=14

[emilyashley]

Making fresh PR so no one has to read through the rebasing :)

https://github.com/honeycombio/libhoney-js/pull/308