Keep GitHub Actions up to date with GitHub's Dependabot

[cclauss]

actions/checkout is @v4, not @v2 same for actions/setup-node, etc.

• https://github.com/search?q=repo%3AWandalen%2Fwretry.action%20checkout&type=code

• https://docs.github.com/en/code-security/dependabot/working-with-dependabot/keeping-your-actions-up-to-date-with-dependabot
• https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#package-ecosystem