Migrate workflows to use Node 20

[meiao]

Description

GHA is migrating its workflows to use Node 20 by Spring 2024. This will break some external actions that we use, so they'll need to be updated.
This will likely affect all repositories, not just the newrelic-java-agent.

Acceptance Criteria

No warnings about Node 16 show up in actions.

[workato-integration]

https://new-relic.atlassian.net/browse/NR-163990

[meiao]

Currently there are no warning messages about Node 16.
Also it is not guaranteed that all actions that we use already have a version running with Node 20 at this point.

Announcement about the change:
https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/

Previously, the changes needed were mostly bumping up versions of the actions: checkout and setup-java.

[meiao]

https://github.blog/changelog/2024-03-07-github-actions-all-actions-will-run-on-node20-instead-of-node16-by-default/

Looks like the date they'll start enforcing Node 20 use is May 13.