CI: update downstream libaec dependency to canonical GitHub repo

[Copilot]

The downstream-ci / metkit (gnu@debian-11) job failed during dependency resolution/build because libaec was referenced via an outdated repository owner. This PR updates the dependency source to the canonical upstream repo used by current GitHub references.

• Root cause

  • Downstream CI dependency resolution for libaec hit a repository-head lookup failure (Not Found) due to stale owner path in CI config.

• Change

  • Updated .github/ci-config.yml dependency entry:
    • MathisRosenhauer/libaec@refs/tags/v1.1.3 → Deutsches-Klimarechenzentrum/libaec@refs/tags/v1.1.3

• Why this is minimal

  • Keeps version/tag unchanged (v1.1.3)
  • Only changes repository path used by downstream workflow dependency fetch logic

dependencies: |
  ecmwf/ecbuild
  Deutsches-Klimarechenzentrum/libaec@refs/tags/v1.1.3
  ecmwf/eccodes
  ecmwf/eckit