lean dependabot

[Seasawher]

Thank you for good app.
This is a feature request.

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Summary: notify Lean update to repo owner

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Users can select "watch level" for upstream version.

• always up-to-date: Automatically open an issue when project's leantoolchain is behind to the latest stable release.
• broken: notify only when nightly build failed
• ignore: reject notifications

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other specs are:

• archived repository is ignored

[Seasawher]

@oliver-butterley created nice GitHub Action: https://github.com/oliver-butterley/lean-update-action

In my opinion, actions that prompt such automatic updates should be performed automatically unless specifically rejected.
Because Lean is still under active development, with monthly releases.