Disables LTS upgrade prompt for Focal

We don't want the OS to nag admins, since we want to ensure the next LTS
is fully supported by SecureDrop before recommending an in-place
upgrade. Disable the prompt on Focal, same as we've done for Xenial in
the past.

Closes #5782.

install_files/securedrop-config-focal/DEBIAN/postinst

@@ -4,8 +4,17 @@
 set -e
 set -x
 
+# Issue #5782
+# Set Prompt=never for distro upgrades
+update_release_prompt() {
+    set -e
+    upgrade_config='/etc/update-manager/release-upgrades'
+    sed -i 's/Prompt=.*/Prompt=never/' "$upgrade_config"
+}
+
 case "$1" in
     configure)
+    update_release_prompt
     # Configuration required for unattended-upgrades
     cp /opt/securedrop/20auto-upgrades /etc/apt/apt.conf.d/
     cp /opt/securedrop/50unattended-upgrades /etc/apt/apt.conf.d/