Remove obsolete downstream dependency updater

[anth-volk]

Fixes #921

Summary

• remove the UK release helper that cloned downstream API repositories and
  invoked scripts those repositories deleted in March 2026
• remove the publication-workflow action that called the obsolete helper
• leave package publication, tagging, and downstream-owned dependency update
  workflows unchanged

PolicyEngine API already updates its own policyengine[models] bundle.
PolicyEngine/policyengine-household-api#1660 tracks adding UK to the Household
API's repository-owned dependency workflow.

Validation

• uv run --frozen --extra dev ruff format . — 998 files unchanged
• uv run --frozen --extra dev ruff check . — passed
• parsed .github/workflows/versioning.yaml with PyYAML
• git diff --check — passed

make format could not find ruff on the shell path, so the same configured
formatter and lint checks were run through the repository's uv development
environment.