ci: use Concierge to set up the smoke test environments (#1541)

The smoke tests are currently failing because of `sg` behaviour changes
in the `ubuntu:latest` runners. Rather than figure out what the solution
for that is, this PR simplifies the entire environment setup to just use
Concierge instead.

This also switches from testing against Juju 3.5 to 3/latest (which is
3.6), since 3.6 is available now and the intention was always to test
against the latest 3.

A [run can be seen in my
fork](https://github.com/tonyandrewmeyer/operator/actions/runs/13003299600).

.github/workflows/smoke.yaml

@@ -13,47 +13,16 @@ jobs:
       matrix:
         # pylibjuju does not currently support Juju 4.x
         # The smoke tests do not yet work on Juju 2.9.
-        juju-version: ['3.5']
-        charmcraft-version: ['2.x', '3.x']
-        cloud: ['lxd', 'microk8s']
-
-    env:
-      JUJU_VERSION: "${{ matrix.juju-version }}"
+        juju-channel: ['3/stable']
+        charmcraft-channel: ['2.x/stable', '3.x/stable']
+        preset: ['machine', 'microk8s']
 
     steps:
-      # LXD is required for charmcraft to pack, even if it's not used as the
-      # Juju cloud.
-      - name: Set up LXD
-        uses: canonical/setup-lxd@54a5806e490d92e207b57183cf111ed54bbdeff4
-        with:
-          channel: 5.0/stable
-
-      - name: Set up Microk8s
-        if: matrix.cloud == 'microk8s'
-        uses: balchua/[email protected]
-        with:
-          channel: '1.26-strict/stable'
-          devMode: 'true'
-          addons: '["dns", "hostpath-storage"]'
-
-      - name: Set up Juju (classic)
-        if: matrix.juju-version == '2.9'
-        run: sudo snap install juju --classic --channel=${{ matrix.juju-version }}
-
-      - name: Set up Juju
-        if: matrix.juju-version != '2.9'
-        run: sudo snap install juju --channel=${{ matrix.juju-version }}
-
-      - name: Bootstrap Juju controller (k8s)
-        if: matrix.cloud == 'microk8s'
-        run: sg snap_microk8s -c 'juju bootstrap microk8s'
-
-      - name: Bootstrap Juju controller (lxd)
-        if: matrix.cloud == 'lxd'
-        run: juju bootstrap localhost
+      - name: Install concierge
+        run: sudo snap install --classic concierge
 
-      - name: Install charmcraft
-        run: sudo snap install charmcraft --channel=${{ matrix.charmcraft-version }} --classic
+      - name: Install Juju and tools
+        run: sudo concierge prepare --juju-channel=${{ matrix.juju-channel }} --charmcraft-channel=${{ matrix.charmcraft-channel }} -p "${{ matrix.preset }}"
 
       - name: Checkout the repository
         uses: actions/checkout@v4