Upgrade Python to 3.14 in python_runtime and python_development images

- Update PYTHON_RUNTIME_VERSION from 3.12-u24.04-1 to 3.14-u24.04-1
- Switch from ubuntu/python base image to ubuntu:24.04 + deadsnakes PPA
  (ubuntu/python does not have Python 3.14 yet)
- Simplify Dockerfile.python_runtime by removing obsolete .deb downloads
- Use update-alternatives to set python3.14 as default python3
- Update documentation in docs/common-ci-images.md

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>