Keep GitHub Actions up-to-date with dependabot

[pnacht]

I noticed that many of the GitHub Actions used by Jansson are a few major versions out-of-date (i.e. actions/checkout@v2 when there's now v4). A dependency update tool such as dependabot can help keep these Actions up-to-date, giving Jansson access to new features/optimizations that may have been released.

I'll send a PR updating the Actions and setting up dependabot.

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Disclosure: My name is Pedro and I work with Google and the Open Source Security Foundation (OpenSSF) to improve the supply-chain security of the open-source ecosystem.