Migrate CI from Ubuntu 20.04

### Context

Because of the [Ubuntu 20.04 deprecation](https://github.com/canonical/bundle-kubeflow/issues/1230) we'll need to move away from 20.04 in this and all CKF repos.

This will affect the CI, that is still relying on 20.04 and Python 3.8 for different actions (building, releasing, testing the charms).

Ideally we should migrate from 20.04 in all aspects of our charms (i.e. bases, python version etc) but this will be a significant undertaking. Thus for now we'll focus on:
1. Using the `actions/setup-python` to install Python 3.8 (affects global `pip` and `python3`). See example [here](https://github.com/canonical/microk8s/blob/0d619ff1c09dea97d9924a9604db263c639bcb50/.github/workflows/build-installer.yml#L20-L23)
2. Update the CI to use Ubuntu 24.04
3. Update the `publish.yaml` and/or `release.yaml` to install LXD and not use destructive mode, since we are now building on a different Ubuntu version than the host, [example PR](https://github.com/canonical/resource-dispatcher/pull/90)

### What needs to get done

1. Use the `actions/setup-python` to install Python 3.8, [example](https://github.com/canonical/microk8s/blob/0d619ff1c09dea97d9924a9604db263c639bcb50/.github/workflows/build-installer.yml#L20-L23)
2. Replace `runs-on: ubuntu-20.04` with `runs-on: ubuntu-24.04`, in the `.github/workflows`
3. Update the `publish.yaml` and/or `release.yaml` to install LXD and not use destructive mode, since we are now building on a different Ubuntu version than the host

### Definition of Done

1. The CI of the repo is not using Ubuntu 20.04 anywhere
2. The changes are merged to the `main` branch