Switch Heroku-24 to the deb822 APT sources format (#255)

Since the upstream Ubuntu 24.04 image's default APT sources list
now uses a new file location and APT's deb822 sources format.

Fixes the warnings of form:
`Target Packages (...) is configured multiple times in /etc/apt/sources.list`

See:
https://bugs.launchpad.net/ubuntu/+source/livecd-rootfs/+bug/2048129
https://discourse.ubuntu.com/t/spec-apt-deb822-sources-by-default/29333/1

Fixes #254.
GUS-W-15213103.

heroku-24/setup.sh

@@ -8,14 +8,36 @@ set -x
 
 export DEBIAN_FRONTEND=noninteractive
 
-# The default sources list minus backports, restricted and multiverse.
-cat >/etc/apt/sources.list <<EOF
-deb [arch=amd64] http://archive.ubuntu.com/ubuntu/ noble main universe
-deb [arch=amd64] http://archive.ubuntu.com/ubuntu/ noble-security main universe
-deb [arch=amd64] http://archive.ubuntu.com/ubuntu/ noble-updates main universe
-deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports/ noble main universe
-deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports/ noble-security main universe
-deb [arch=arm64] http://ports.ubuntu.com/ubuntu-ports/ noble-updates main universe
+# This is the default `ubuntu.sources` from both the AMD64 and ARM64 images
+# combined, with `noble-backports`, `restricted` and `multiverse` removed.
+cat >/etc/apt/sources.list.d/ubuntu.sources <<EOF
+Types: deb
+URIs: http://archive.ubuntu.com/ubuntu/
+Suites: noble noble-updates
+Components: main universe
+Signed-By: /usr/share/keyrings/ubuntu-archive-keyring.gpg
+Architectures: amd64
+
+Types: deb
+URIs: http://security.ubuntu.com/ubuntu/
+Suites: noble-security
+Components: main universe
+Signed-By: /usr/share/keyrings/ubuntu-archive-keyring.gpg
+Architectures: amd64
+
+Types: deb
+URIs: http://ports.ubuntu.com/ubuntu-ports/
+Suites: noble noble-updates
+Components: main universe
+Signed-By: /usr/share/keyrings/ubuntu-archive-keyring.gpg
+Architectures: arm64
+
+Types: deb
+URIs: http://ports.ubuntu.com/ubuntu-ports/
+Suites: noble-security
+Components: main universe restricted multiverse
+Signed-By: /usr/share/keyrings/ubuntu-archive-keyring.gpg
+Architectures: arm64
 EOF
 
 apt-get update