ci: use ubuntu-latest; bump checkout and python versions (following ops)

[james-garner-canonical]

This PR updates the build-and-test workflow to run on ubuntu-latest instead of ubuntu-22.04, following ops.

It also bumps the versions on the checkout and setup-python actions accordingly, and pins the tox version following ops.

For the integration tests, which require running tox as root, we install tox via apt.

Also bump the juju version from 3.1 to 3.6 since it's the LTS release.