Upgrade node, rush, and pnpm to latest

[mikeharder]

• Fixes #681
• Upgrade minimum Node version to 18
  • package.json/engines
  • rush.json
• Upgrade default Node version to LTS / 20
  • CONTRIBUTING.md
  • pipelines
• Upgrade rush from 5.62.1 to 5.121.0
  • Update generated "common/scripts" folder
• Upgrade pnpm from 6.22.1 to 8.15.7
  • Upgrade pnpm-lock.yaml from v5 to v6
  • pnpm@9 released last week, but too soon to upgrade
• Includes "rush update --full"
  • Recommended to upgrade all deps when upgrading node/rush/pnpm
• In tests, sort results before comparing to ensure stable order

Open Questions

• Should we publish a release of these packages with this upgrade? Or just consider this an infrastructure upgrade, and wait until the next time someone wants to publish the packages for another reason?

[konrad-jamrozik]

Re publishing: I am ok either way. Let's wait for now?

Also, here are the recent instruction:

• https://github.com/Azure/azure-openapi-validator/issues/683