fix: use secrets.GITHUB_TOKEN instead of custom org secret for release job

[irfanuddinahmad]

Summary

• `release.yml`'s `python-semantic-release` step used `secrets.OPENEDX_SEMANTIC_RELEASE_GITHUB_TOKEN`, a custom org secret. `openedx/sample-plugin`'s current `release.yml` (the org's reference implementation) uses `secrets.GITHUB_TOKEN` instead, and the custom secret isn't needed.
• This follows directly from feanil's review comment on event-routing-backends#571, which flagged the same divergence during the broader Python-tooling-modernization effort (Modernize Python repos: pyproject.toml + uv + semantic-release public-engineering#506).

Test plan

• YAML validated (`python3 -c "import yaml; yaml.safe_load(...)"`)
• Diffed against `openedx/sample-plugin`'s current `release.yml` to confirm the line now matches

[openedx-webhooks]

Thanks for the pull request, @irfanuddinahmad!

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