Add dependabot GitHub action to automatically re-generate scratch env go.mod files

[sbueringer]

As the title says. Let's add a dependabot GitHub action to automatically re-generate go.mod.

Otherwise we probably will have to fix them up ourselves, like e.g. in #2441

Example implementation: https://github.com/kubernetes-sigs/cluster-api/blob/main/.github/workflows/pr-dependabot.yaml

[sbueringer]

/assign

[sbueringer]

Done in #2447 and #2448

/close

[k8s-ci-robot]

@sbueringer: Closing this issue.

In response to this:

Done in #2447 and #2448

/close

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