Added dependabot for GitHub Actions

[ydah]

How about using dependabot in this way?
When this works, a PullRequest is created as follows:

• Bump actions/checkout from 2 to 3 ruby/bigdecimal#242

It was created with reference to the following:

• Added dependabot for GitHub Actions ruby/csv#265

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• [-] Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• [-] Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• [-] Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.