Keep GitHub Actions up to date with GitHub's Dependabot

[cclauss]

Fixes software supply chain safety warnings like at the bottom right of https://github.com/bytecodealliance/wasmtime-py/actions/runs/8654982733

• Keeping your actions up to date with Dependabot
• Configuration options for the dependabot.yml file - package-ecosystem

[alexcrichton]

Thanks!

[cclauss]

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