chore: upgrade Go to 1.27.3 and Atlas to 1.3.4
Merged
MI
2d ago+11+11+01234567890123456789-11-11-01234567890123456789
Upgrade the main Go module and the CLI, Control Plane, and Artifact CAS release images to Go 1.27.3. Update the documented Go version to match.
Upgrade Atlas to 1.3.4 across the migration image, development installer, and CI. Pin both Go and Atlas images to their official multi-architecture digests.
Assisted-by: Codex