directly support ubuntu based distros in add repository snippets

default to VERSION_CODENAME but use UBUNTU_CODENAME if present and remove comment to do so manually

Author: nisrael

content/manuals/engine/install/ubuntu.md

@@ -127,16 +127,11 @@ Docker from the repository.
    # Add the repository to Apt sources:
    echo \
      "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/docker.asc] {{% param "download-url-base" %}} \
-     $(. /etc/os-release && echo "$VERSION_CODENAME") stable" | \
+     $(. /etc/os-release && echo "${UBUNTU_CODENAME:-$VERSION_CODENAME}") stable" | \
      sudo tee /etc/apt/sources.list.d/docker.list > /dev/null
    sudo apt-get update
    ```
 
-   > [!NOTE]
-   >
-   > If you use an Ubuntu derivative distribution, such as Linux Mint,
-   > you may need to use `UBUNTU_CODENAME` instead of `VERSION_CODENAME`.
-
 2. Install the Docker packages.
 
    {{< tabs >}}