ci: switch to team PPA for Azure VPN (#1029)

I've repackaged `ppp` and `sstp-client` in the team PPA for both Jammy
and Noble, so that we are not affected by when the `ubuntu-latest`
runner transitions to Noble on GitHub Actions.

Here's a passing run with `ubuntu-latest` (aka 22.04):
https://github.com/GabrielNagy/adsys/actions/runs/9502739724
And with `ubuntu-24.04`:
https://github.com/GabrielNagy/adsys/actions/runs/9506065559

Fixes UDENG-3048

.github/actions/azure-sstpc-vpn/action.yaml

@@ -25,7 +25,7 @@ runs:
     - name: Install required packages from PPA
       shell: bash
       run: |
-        sudo add-apt-repository -y ppa:gabuscus/ppp
+        sudo add-apt-repository -y ppa:ubuntu-enterprise-desktop/ppp
         sudo apt-get update
         sudo DEBIAN_FRONTEND=noninteractive apt-get install -y sstp-client
     - name: Write certificate data to disk