Remove redundant bundle update from CI

[nertzy]

Summary

• Remove the explicit bundle update --all step from CI.
• Rely on ruby/setup-ruby with bundler-cache: true, which already generates a lockfile with the newest compatible gems and runs bundle install.

Test plan

• Parse .github/workflows/ci.yml as YAML.
• Run the no-lockfile bundle lock and bundle install path with Ruby 4, Bundler 4, and Active Record 8.1.
• Run the RSpec and Minitest suites: 133 examples and 10 runs, 0 failures.