Modernize APT configuration format.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

package.json

@@ -89,8 +89,8 @@
       "synopsis": "Zulip Desktop App",
       "afterInstall": "./packaging/deb-after-install.sh",
       "fpm": [
-        "./packaging/deb-apt.list=/etc/apt/sources.list.d/zulip-desktop.list",
-        "./packaging/deb-apt.asc=/etc/apt/trusted.gpg.d/zulip-desktop.asc",
+        "./packaging/deb-apt.sources=/etc/apt/sources.list.d/zulip-desktop.sources",
+        "./packaging/deb-apt.asc=/usr/share/keyrings/zulip-desktop.asc",
         "./packaging/deb-release-upgrades.cfg=/etc/update-manager/release-upgrades.d/zulip-desktop.cfg"
       ]
     },

packaging/deb-after-install.sh

@@ -11,3 +11,6 @@ update-desktop-database /usr/share/applications || true
 
 # Clean up configuration for old Bintray repository
 rm -f /etc/apt/zulip.list
+
+# Clean up legacy APT configuration
+rm -f /etc/apt/sources.list.d/zulip-desktop.list /etc/apt/trusted.gpg.d/zulip-desktop.asc

packaging/deb-apt.list

@@ -1 +0,0 @@
-deb https://download.zulip.com/desktop/apt stable main

packaging/deb-apt.sources

@@ -0,0 +1,5 @@
+Types: deb
+URIs: https://download.zulip.com/desktop/apt/
+Suites: stable
+Components: main
+Signed-By: /usr/share/keyrings/zulip-desktop.asc