fix: use charmcraft 2.x to build

Version 3 of charmcraft does not allow a run-on stanza that includes
24.04 with other versions, but 2 does. So we install this versions for
   our Github workflows.

.github/workflows/ci.yml

@@ -93,7 +93,7 @@ jobs:
         env:
           CHARMCRAFT_AUTH: ${{ secrets.CHARMCRAFT_AUTH }}
         run: |
-          sudo snap install charmcraft --classic
+          sudo snap install charmcraft --channel 2.x/stable --classic
           charmcraft upload ${{ steps.download.outputs.download-path }}/*.charm \
             --name $CHARM_NAME \
             --release ${{ needs.channel.outputs.test }}
@@ -179,7 +179,7 @@ jobs:
     steps:
     - name: Install Charmcraft
       run: |
-        sudo snap install charmcraft --classic
+        sudo snap install charmcraft --channel 2.x/stable --classic
 
     - name: Get uploaded revision
       id: revision