ci: upgrade actions to v6 and add py3.14 to test list

[waketzheng]

Description

1. Upgrade actions/checkout and actions/setup-python to their latest versions.
2. Add Python 3.14 to the CI test matrix and declare it as supported in pyproject.toml.

[waketzheng]

@elapouya, without upgrading the actions, the CI cannot run successfully:

Node 20 is being deprecated. This workflow is running with Node 24 by default. If you need to temporarily use Node 20, you can set the ACTIONS_ALLOW_USE_UNSECURE_NODE_VERSION=true environment variable. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/
Run actions/setup-python@v1
  with:
    python-version: 3.9
    architecture: x64
Error: Version 3.9 with arch x64 not found
Available versions:

3.10.20 (x64)
3.11.15 (x64)
3.12.13 (x64)
3.13.14 (x64)
3.14.6 (x64)