dependabot: Group Sorbet updates together

[issyl0]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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• A PR every day for each of sorbet, sorbet-runtime and sorbet-static-and-runtime is a bit much.
• Based on https://github.com/dependabot/dependabot-core/blob/6e4d9d8532939a683168e59b0620d4c2b1e33621/.github/dependabot.yml#L12-L17.

[p-linnane]

Thanks so much for this @issyl0! Looks good: #18982

[dduugg]

💯 Amazing stuff!