Update Slack SDK release candidates

[slack-samples]

Type of change

• Dependency update

Summary

Updates this sample for the Slack npm package release candidates:

• @slack/bolt 5.0.0-rc.1 where used directly
• @slack/types 3.0.0-rc.1 where used directly
• transitive Slack SDK packages in the lockfile resolve to the requested RC line through Bolt v5

Requirements

• I have ensured the changes I am contributing align with existing patterns and have tested and linted my code
• I've read and agree to the Code of Conduct

Testing

• block-kit/: npm ci --ignore-scripts
• block-kit/: npm run lint
• block-kit/: npm run check
• block-kit/: npm run test

Reviewers

Please verify the sample can still be created and run with Slack CLI using this branch:

$ slack create -t slack-samples/bolt-js-examples --subdir block-kit --branch claw/update-slack-sdk-rcs-2026-05-19