Add dependabot configuration file

[ghost]

Merge Checklist

All boxes should be checked before merging the PR (just tick any boxes which don't apply to this PR)

• The toolchain has been rebuilt successfully (or no changes were made to it)
• The toolchain/worker package manifests are up-to-date
• Any updated packages successfully build (or no packages were changed)
• Package tests (%check section) have been verified with RUN_CHECK=y for existing SPEC files, or added to new SPEC files
• All package sources are available
• cgmanifest files are up-to-date and sorted (./cgmanifest.json, ./toolkit/tools/cgmanifest.json, ./toolkit/scripts/toolchain/cgmanifest.json, .github/workflows/cgmanifest.json)
• LICENSE-MAP files are up-to-date (./SPECS/LICENSES-AND-NOTICES/data/licenses.json, ./SPECS/LICENSES-AND-NOTICES/LICENSES-MAP.md, ./SPECS/LICENSES-AND-NOTICES/LICENSE-EXCEPTIONS.PHOTON)
• All source files have up-to-date hashes in the *.signatures.json files
• sudo make go-tidy-all and sudo make go-test-coverage pass
• Documentation has been updated to match any changes to the build system
• Ready to merge

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Summary

Dependabot is a useful tool to keep dependencies up-to-date. However, the default settings don't reflect our repository's workflow with respect to default reviewer sets and target branches (e.g. we don't PR against the default branch, usually). This PR sets up a configuration file to allow the tools to work better for us.

Cherry-pick of #1197 for 1.0 branch.

Change Log

• Run dependency checks daily at 7AM Redmond time
• Use microsoft/cbl-mariner-devs group as a default reviewer for dependabot PRs
• Change dependabot target branch to 1.0-dev
• Run checks against our go modules and our github actions

Does this affect the toolchain?

NO

Test Methodology

• The configuration file was validated using the tool's configuration validator.