Upgrade kitconcept.core to 2.0.0b2 and Python to 3.14

[ericof]

Summary

Upgrades the distribution to kitconcept.core 2.0.0b2 (backend and frontend) and bumps the supported Python version to 3.14.

Changes

• Update Python version to 3.14 — CI workflow default (.github/workflows/config.yml) and requires-python / classifiers in backend/pyproject.toml.
• Upgrade kitconcept.core to 2.0.0b2 (backend) — dependency + constraint bumps in backend/pyproject.toml, refreshed backend/uv.lock, and backend/version.txt → 2.0.0b2. Pulls in Plone 6.2.1 and updated Plone stack constraints.
• Refactor create_site script — backend/scripts/create_site.py and backend/src/kitconcept/website/utils/scripts.py now use helper functions provided by kitconcept.core, removing the now-redundant local implementation and its test (backend/tests/utils/test_scripts.py).
• Upgrade @kitconcept/core to 2.0.0-beta.2 (frontend) — frontend/mrs.developer.json tag bump and refreshed frontend/pnpm-lock.yaml.

Related

Tracked in GitLab issue #70.

Notes

• News fragments added for each scope (root / backend / frontend).