FIX-#0000: Add a Dependabot config to auto-update GitHub action versions

[kurtmckee]

What do these changes do?

Currently, deprecation warnings are being thrown due to actions using out-of-date Node versions (recent example). Rather than submitting a PR to update them manually, this PR configures Dependabot to regularly submit PRs to update GitHub action versions as needed.

If this PR merges, you can expect Dependabot to immediately submit a PR to update actions to newer versions.

• first commit message and PR title follow format outlined here

  NOTE: If you edit the PR title to match this format, you need to add another commit (even if it's empty) or amend your last commit for the CI job that checks the PR title to pick up the new PR title.

• passes flake8 modin/ asv_bench/benchmarks scripts/doc_checker.py
• passes black --check modin/ asv_bench/benchmarks scripts/doc_checker.py
• signed commit with git commit -s
• Resolves #?
• tests added and passing
• module layout described at docs/development/architecture.rst is up-to-date

[devin-petersohn]

Thanks @kurtmckee, LGTM