Add Dependabot workflow to update GitHub Actions

[jcfr]

• Pin GitHub actions to full length commit SHA.

  • GitHub's security hardening guide recommends this mitigation method.
  • See https://docs.github.com/en/actions/security-guides/security-hardening-for-github-actions#using-third-party-actions

• Add Dependabot workflow to update GitHub Actions:

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

Adapted from work done by @jamesobutler in:

• https://github.com/Slicer/Slicer/pull/6888

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