Refresh v5 Ruby image, remove Pry, and update checkout

[crohr]

Context

Refresh the v5 action image after the Debian Bullseye repository expiry reported in #140, and update the preview workflows to current checkout.

Root cause

The Dockerfile still pins a 2023 image for an OpenSSH restriction whose documented deadline has passed. The provider already creates Amazon Linux 2023 instances. The old Pry dependency also crashes at startup on Ruby 3.3, although it only supports an unused debug console.

What changed

• Use ruby:3.3-slim-bookworm and remove expired compatibility comments.
• Remove Pry, its two dependencies, and the unused console command.
• Update all three workflow checkout steps to actions/checkout@v7 (latest stable release: v7.0.1).

Validation

• Built the action image for Linux amd64; apt update and bundle install passed.
• Ran up --help, github-sync --help, and bundle check in the image.
• Loaded the Lightsail SDK and completed a stubbed get_instances request; confirmed Pry is absent.
• git diff --check passed. make test is a no-op on v5; this Ruby branch has no Go dist target.
• Final adversarial review found no substantive issues, including architectural complexity.
• Final deployment passed in 1m6s for 40244b606595e2c7f6ec090a1144b1b4bd8d3f20, including checkout v7.
• Rechecked the live preview: HTTP 200 and WordPress installation page.
• The pullpreview label remains applied.