ci: use workflow token for release please

[IceRhymers]

Summary

• run release-please with the workflow-scoped GITHUB_TOKEN
• retain the existing explicit contents: write and pull-requests: write permissions
• remove the dependency on the under-permissioned GH_TOKEN secret that failed at Git Database tree creation

Verification

• make check
• make test — 171 passed, 8 deselected

The merge to master will let release-please create the pending chore: release 0.5.1 PR from the unreleased fix: commit.