deps: exclude core from auto update

[wasaga]

Summary

We want IC main to generally track main in core, and often updates are synchronized to reflect any changes in core. This PR excludes pomerium/pomerium from dependabot Go group updates.

Related issues

Checklist

• [ ] reference any related issues
• [ ] updated docs
• [ ] updated unit tests
• [ ] updated UPGRADING.md
• [ ] add appropriate tag (improvement / bug / etc)
• [ ] ready for review