Add Julia ecosystem to Dependabot configuration

[ChrisRackauckas-Claude]

Summary

This PR adds Julia ecosystem support to the existing Dependabot configuration and removes CompatHelper.

Changes:

• Updated .github/dependabot.yml to include Julia ecosystem (beta feature)
• Removed .github/workflows/CompatHelper.yml

Benefits:

• Native GitHub Dependabot now handles Julia dependencies
• No need for separate CompatHelper workflow
• Groups all Julia package updates into a single PR

Configuration

The updated Dependabot configuration now includes:

• Updates GitHub Actions weekly (existing)
• Updates Julia dependencies daily (new)
• Groups all Julia package updates into a single PR (new)

References

• Dependabot documentation
• Example: https://github.com/JuliaDiff/DifferentiationInterface.jl/blob/main/.github/dependabot.yml

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