Update automated unit tests to run on Node 20

[gavinr-maps]

Describe the problem

Node v20 is now the current active LTS version: https://github.com/nodejs/release#release-schedule

Describe the proposed solution

So we should probably add v20 to the list of versions that we automatically test on:

https://github.com/Esri/arcgis-rest-js/blob/9ec89a2113dd616519398cc217309930fed7c7bc/.github/workflows/pull-request-tests.yml#L21

Alternatives considered

No response

Additional Information

Questions:

• Should we also remove Node 14 from the automated unit tests (end of life was 2023-04-30)?
• Should we also remove Node 16 from the automated unit tests (end of life was 2023-09-11)?

[patrickarlt]

I think we should remove 14 and 16.

[0scvr]

I would like to work on this issue, please :)

[patrickarlt]

@0scvr all yours!

[gavinr-maps]

Updated in #1144 - thanks @0scvr and @patrickarlt!