Cleanup Master CI

• update:
  • charmcraft.yaml for v3
  • tox.ini to use py3.12 by default
  • test-requirements.in to use zaza for juju3
  • update test bundles to work with charmcraft v3
• generate:
  • merged-requirements-py312.txt
  • test-requirements-py312.txt
• add:
  • update-requirements env to tox.ini and src/tox.ini
• remove:
  • build.lock