Update major Dependabot dependencies

[djalmaaraujo]

Summary

• apply the remaining major Dependabot updates on top of Update non-major Dependabot dependencies #376
• update gem minitest to 6.0.6 and tailwind_merge to 1.4.0
• update docs puma to 8.0.1 and minitest to 6.0.6
• update GitHub Actions checkout/pnpm setup major versions
• update the docs devcontainer node feature to v2

Included Dependabot PRs

• Supersedes Bump pnpm/action-setup from 5 to 6 #360
• Supersedes Bump actions/checkout from 4 to 6 #361
• Supersedes Bump ghcr.io/devcontainers/features/node from 1.7.1 to 2.0.0 in /docs #363
• Supersedes Update tailwind_merge requirement from ~> 0.12 to ~> 1.4 in /gem #367
• Supersedes Bump puma from 7.2.0 to 8.0.1 in /docs #370
• Supersedes Update minitest requirement from ~> 5.0 to ~> 6.0 in /gem #371
• Supersedes Bump minitest from 6.0.4 to 6.0.6 in /docs #373

Stacking / conflict analysis

• This PR is based on Update non-major Dependabot dependencies #376 because the major updates touch the same gem and docs lockfiles as the non-major updates.
• Keeping it stacked makes the diff only the major-version delta and avoids duplicated or conflicting Bundler lock resolution.
• After Update non-major Dependabot dependencies #376 merges, this PR can be retargeted to main or rebased onto main.

Verification

• docker Ruby 3.4.7: bundle exec rake test
• docker Ruby 3.4.7: bundle exec rake standard
• docker Ruby 3.4.7 + Node 20/pnpm 10.8.0: bundle exec standardrb
• docker Ruby 3.4.7 + Node 20/pnpm 10.8.0: bundle exec rails db:test:prepare test