ci: add OSS Exit Gate release workflow and remove legacy release-trigger

[dborowitz]

Summary

Adds the OSS Exit Gate PyPI release workflow (.github/workflows/release.yaml) and retires the legacy .github/release-trigger.yml Kokoro trigger:

• OSS Exit Gate Release Workflow (.github/workflows/release.yaml):
  • Triggered on release: types: [published] (published by release-please) or manual workflow_dispatch with an explicit tag input.
  • Builds sdist and wheel via python3 -m build, validates package metadata with twine check dist/*, authenticates via Workload Identity Federation (projects/305452601764/locations/global/workloadIdentityPools/builders/providers/github), uploads distributions to Artifact Registry (https://us-python.pkg.dev/oss-exit-gate-prod/google-cloud-spark-connect--pypi), and writes publish_all.json to trigger OSS Exit Gate publication to PyPI.
  • Hardened with explicit top-level permissions: { contents: read, id-token: write }, persist-credentials: false, enable-cache: false on setup-uv, concurrency serialization, timeout-minutes: 15, and full 40-char commit SHA action pins.
• Release-Please Configuration (.github/release-please.yml & .github/release-trigger.yml):
  • Removes the obsolete dataproc-spark-connect-v0 branch entry from .github/release-please.yml.
  • Deletes .github/release-trigger.yml (enabled: true), which previously triggered the legacy Kokoro autorelease pipeline.

[gemini-code-assist]

Code Review

This pull request simplifies the .github/release-please.yml configuration by removing the specific branch configuration for dataproc-spark-connect-v0. Additionally, an unnamed configuration file has been cleaned up by removing the enabled: true line. There are no review comments, and I have no feedback to provide.