Add Dependabot configuration for GitHub Actions

[Copilot]

Adds automated dependency management for GitHub Actions to keep workflow dependencies up to date with security patches and new features.

Changes

• Created .github/dependabot.yml with weekly update schedule for GitHub Actions
• Configured to monitor all workflows: CI, documentation, TagBot, and CompatHelper
• Set limit of 10 concurrent PRs to prevent overwhelming maintainers

Monitored Actions

Dependabot will now track updates for:

• actions/checkout@v4
• julia-actions/* (setup-julia, cache, buildpkg, runtest, processcoverage)
• codecov/codecov-action@v5
• JuliaRegistries/TagBot@v1

Dependabot will automatically create PRs with changelogs when updates are available.

Original prompt

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This section details on the original issue you should resolve

<issue_title>CI: Set up Dependabot configuration for GitHub Actions</issue_title>
<issue_description>Set up Dependabot to automatically check for and update dependencies.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes CI: Set up Dependabot configuration for GitHub Actions #373

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