Replace Node.js 18 with Node.js 20

[dcroote]

Rationale: Slack discussion and Node.js end of life timeline.

Update in:

• package.json
• Dockerfiles
• GitHub Actions workflows
• Terraform (AWS & GCP)

[Siegrift]

There is a lot of discussion happening on the PR and the problem is that the E2E tests are failing. It's not great, because node@18 receives only security updates. After 30 Apr 2025, it completely stops being maintained.

Let's revisit this issue after some time.