chore: Support Node 18+ (googleapis#2540)

* feat: Support Node 18+

* chore: Additional Node 18 upgrades

* chore: don't change test fixtures

* feat: Move kokoro directories to 18

* chore: Update checkout and setup-node to v4

- https://github.com/actions/checkout/releases/tag/v4.0.0

- https://github.com/actions/setup-node/releases/tag/v4.0.0

Source-Link: googleapis/synthtool@c19dd80
Post-Processor: gcr.io/cloud-devrel-public-resources/owlbot-nodejs:latest@sha256:8060aba1f6d5617d08091767141ab2a99ea1ccbd9371fd42ffc208c5329caa73

Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>
Co-authored-by: Denis DelGrosso <85250797+ddelgrosso1@users.noreply.github.com>

Author: gcf-owl-bot[bot]