Drop Python v3.8 support

Date when Python project won’t support v3.8

• Python 3.8 support will be dropped in October 14th of 2024 [1]
• Platform which still uses Py3.8

Todos

DuniterPy

• pyproject.toml: Bump minimum versions:
  • tools.poetry.dependency.python
  • .pre-commit-config.yaml:pyupgrade/tools.ruff.target-version
• Delete test job for v3.8 from .gitlab-ci.yml
• Update minimal version in documentation: README.md and docs/index.rst
• Create ticket for Py3.9/3.14: Add Python v3.14 support and drop Python v3.9 (#212)