Mullvad no longer supports Ubuntu 20.04

Mullvad supports the latest 2 LTS releases.

Ubuntu 24.04 came out April 2024.

On 2024-05-16 the installation script started to fail:

> `E: The repository 'https://repository.mullvad.net/deb/stable focal Release' does not have a Release file.`

https://github.com/iainelder/dotfiles/actions/runs/9106964568/job/25035011989

.github/workflows/mullvad-vpn.yml

@@ -19,7 +19,9 @@ jobs:
       fail-fast: false
       matrix:
         os:
-          - "ubuntu:20.04"
+          # Mullvad supports the last two LTS versions.
+          # Ubuntu 24.04 came out April 2024.
+          # - "ubuntu:20.04"
           - "ubuntu:22.04"
 
     runs-on: ubuntu-latest