Update to charmcraft 3 (#116)

Fixes: #95

.github/workflows/check.yaml

@@ -96,7 +96,7 @@ jobs:
 
       - name: install external controller required packages
         run: |
-           sudo snap install charmcraft --channel 2.x/stable --classic
+           sudo snap install charmcraft --channel 3.x/stable --classic
            sudo snap install juju --channel 3.4/stable --classic
            sudo --preserve-env=http_proxy,https_proxy,no_proxy pip3 install tox
            juju switch soleng-ci-ctrl-34

.github/workflows/promote.yaml

@@ -37,4 +37,4 @@ jobs:
           github-token: ${{ secrets.GITHUB_TOKEN }}
           destination-channel: ${{ steps.set-channels.outputs.destination-channel }}
           origin-channel: ${{ steps.set-channels.outputs.origin-channel }}
-          charmcraft-channel: "2.x/stable"
+          charmcraft-channel: "3.x/stable"

.github/workflows/release.yaml

@@ -36,7 +36,7 @@ jobs:
       - name: Pack and upload to charmhub
         uses: canonical/charming-actions/[email protected]
         with:
-          charmcraft-channel: "2.x/stable"
+          charmcraft-channel: "3.x/stable"
           credentials: "${{ secrets.CHARMHUB_TOKEN }}"
           github-token: "${{ secrets.GITHUB_TOKEN }}"
           # Ensure the charm is built in an isolated environment and on the correct base in an lxd container.

CONTRIBUTING.md

@@ -57,6 +57,12 @@ This script is also executed periodically as part of a GitHub Actions workflow t
 
 ## Build the charm
 
+Charmcraft v3 is required to build this charm:
+
+```
+sudo snap install charmcraft --channel 3.x/stable --classic
+```
+
 Build the charm in this git repository using:
 
 ```shell