Adds a dependabot.yml file to auto-upgrade opentofu dependencies

[diofeher]

Resolves #113

Instead of manually upgrading the packages used in dependencies, this PR aims to automatically handle that, starting only with opentofu-schema and tofu-exec, under our org.

Checklist

• I have read the contribution guide.
• I have not used an AI coding assistant to create this PR.
• I have written all code in this PR myself OR I have marked all code I have not written myself (including modified code, e.g. copied from other places and then modified) with a comment indicating where it came from.
• I (and other contributors to this PR) have not looked at the Terraform source code while implementing this PR.
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Go checklist

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• I have run existing tests to ensure my code doesn't break anything.
• I have added tests for all relevant use cases of my code, and those tests are passing.
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