docs: update Ruby version, tutorial notes for multi-containers

[glasnt]

Changes:

• Update Ruby version from 2.7.4 (EOL) to 3.2.2 (latest)
• Add explicit rack server (probably due to major Ruby version update)

Doc updates:

• Removed referenced to "2nd gen" Cloud SQL (not mentioned in docs or UI)
• Added optional note to create a user, or use default "postgres"
• Added API enabling (Cloud Build and Cloud Run prompted in CLI, Cloud SQL Admin isn't, so added all three)
• Added optional note around using Secret Manager rather than hardcoding password as an environment variable, linking to docs
• Added note around how the default service account will have permission to connect to Cloud SQL, but explicit permission required for custom service accounts
• Fixed filename on gcloud run command

Other improvements that can be made (but require more work):

• Container Registry is deprecated, need to update to use Artifact Registry (https://cloud.google.com/artifact-registry/docs/transition/transition-from-gcr)
• Could use Cloud Buildpacks for Ruby instead of Dockerfile, but native gem pg building had issues, would need further investigation.

cc @shubha-rajan for review

Happy to also do the same changes to GoogleCloudPlatform/alloydb-auth-proxy once this is merged.

[jackwotherspoon]

Thanks @glasnt! These look like great improvements!

[jackwotherspoon]

Thanks for this @glasnt ! If you would be so kind to port these same changes over to the alloydb-auth-proxy too that would be greatly appreciated 😃