docs: Update production supported releases.

Now that we have production support for Ubuntu Focal, we update the
documentation to state our support for it.

(We also drop deprecated Xenial and Stretch from supported platforms).

docs/production/requirements.md

@@ -3,10 +3,9 @@
 To run a Zulip server, you will need:
 * A dedicated machine or VM
 * A supported OS:
+  * Ubuntu 20.04 Focal
   * Ubuntu 18.04 Bionic
-  * Ubuntu 16.04 Xenial (deprecated)
   * Debian 10 Buster
-  * Debian 9 Stretch  (deprecated)
 * At least 2GB RAM, and 10GB disk space
   * If you expect 100+ users: 4GB RAM, and 2 CPUs
 * A hostname in DNS
@@ -29,13 +28,11 @@ can't support you, but
 
 #### Operating System
 
-Ubuntu 18.04 Bionic, Debian 10 Buster, Ubuntu 16.04 Xenial (deprecated),
-and Debian 9 Stretch (deprecated) are supported for running Zulip in
-production.  64-bit is recommended.  We also recommend installing on
-the newest option you're comfortable with, to save your organization
-the work of upgrading (Ubuntu Trusty [reached end of life in April
-2019][trusty-eol]; Zulip 2.0 was the last major release to support
-it).
+Ubuntu 20.04 Focal, 18.04 Bionic, and Debian 10 Buster are supported
+for running Zulip in production.  64-bit is recommended.  We recommend
+installing on the newest supported OS release you're comfortable with,
+to save a bit of future work [upgrading the operating
+system][upgrade-os].
 
 If you're using Ubuntu, the
 [Ubuntu universe repository][ubuntu-repositories] must be
@@ -46,6 +43,7 @@ sudo add-apt-repository universe
 sudo apt update
 ```
 
+[upgrade-os]: ../production/upgrade-or-modify.html#upgrading-the-operating-system
 [ubuntu-repositories]:
 https://help.ubuntu.com/community/Repositories/Ubuntu
 [enable-universe]: https://help.ubuntu.com/community/Repositories/CommandLine#Adding_the_Universe_and_Multiverse_Repositories

docs/production/upgrade-or-modify.md

@@ -186,7 +186,7 @@ custom configuration.
 ## Upgrading the operating system
 
 When you upgrade the operating system on which Zulip is installed
-(E.g. Ubuntu 16.04 Xenial to Ubuntu 18.04 Bionic), you need to take
+(E.g. Ubuntu 18.04 Bionic to Ubuntu 20.04 Focal), you need to take
 some additional steps to update your Zulip installation, documented
 below.
 
@@ -194,10 +194,10 @@ The steps are largely the same for the various OS upgrades aside from
 the versions of postgres, so you should be able to adapt these
 instructions for other supported platforms.
 
-### Upgrading from Ubuntu 16.04 Xenial to 18.04 Bionic
+### Upgrading from Ubuntu 18.04 Bionic to 20.04 Focal
 
-1. Upgrade your server to the latest Zulip `2.1.x` release, since
-   newer releases don't support Ubuntu 16.04 Xenial.
+1. Upgrade your server to the latest Zulip release (at least 2.2.0,
+   which adds support for Ubuntu Focal).
 
 2. As the Zulip user, stop the Zulip server and run the following
    to back up the system:
@@ -227,13 +227,13 @@ instructions for other supported platforms.
    match the new OS version:
 
     ```
-    touch /usr/share/postgresql/10/pgroonga_setup.sql.applied
+    touch /usr/share/postgresql/12/pgroonga_setup.sql.applied
     /home/zulip/deployments/current/scripts/zulip-puppet-apply -f
-    pg_dropcluster 10 main --stop
+    pg_dropcluster 12 main --stop
     systemctl stop postgresql
-    pg_upgradecluster 9.5 main
-    pg_dropcluster 9.5 main
-    apt remove postgresql-9.5
+    pg_upgradecluster 10 main
+    pg_dropcluster 10 main
+    apt remove postgresql-10
     systemctl start postgresql
     systemctl restart memcached
     ```
@@ -254,15 +254,44 @@ That last command will finish by restarting your Zulip server; you
 should now be able to navigate to its URL and confirm everything is
 working correctly.
 
+### Upgrading from Ubuntu 16.04 Xenial to 18.04 Bionic
+
+1. Upgrade your server to the latest Zulip `2.1.x` release, since
+   newer releases don't support Ubuntu 16.04 Xenial.
+
+2. Same as for Bionic to Focal.
+
+3. Same as for Bionic to Focal.
+
+4. As root, upgrade the database installation and OS configuration to
+   match the new OS version:
+
+    ```
+    touch /usr/share/postgresql/10/pgroonga_setup.sql.applied
+    /home/zulip/deployments/current/scripts/zulip-puppet-apply -f
+    pg_dropcluster 10 main --stop
+    systemctl stop postgresql
+    pg_upgradecluster 9.5 main
+    pg_dropcluster 9.5 main
+    apt remove postgresql-9.5
+    systemctl start postgresql
+    systemctl restart memcached
+    ```
+
+5. Same as for Bionic to Focal.
+
+That last command will finish by restarting your Zulip server; you
+should now be able to navigate to its URL and confirm everything is
+working correctly.
 
 ### Upgrading from Ubuntu 14.04 Trusty to 16.04 Xenial
 
 1. Upgrade your server to the latest Zulip `2.0.x` release, since newer
    releases don't support Ubuntu 14.04 Trusty.
 
-2. Same as for Xenial to Bionic.
+2. Same as for Bionic to Focal.
 
-3. Same as for Xenial to Bionic.
+3. Same as for Bionic to Focal.
 
 4. As root, upgrade the database installation and OS configuration to
 match the new OS version:
@@ -279,7 +308,7 @@ match the new OS version:
     service memcached restart
     ```
 
-5. Same as for Xenial to Bionic.
+5. Same as for Bionic to Focal.
 
 ### Upgrading from Debian Stretch to Debian Buster