Update Python version matrix in workflow documentation (#35529)

- Remove Python 3.8 (no longer supported)
- Add Python 3.12 (now supported)
- Update all workflow documentation entries to match gradle.properties
- Affects 21+ instances in .github/workflows/README.md

This aligns the workflow documentation with the official Python
version support defined in gradle.properties: 3.9,3.10,3.11,3.12,3.13

Author: BorisQuanLi