help: Document upgrading Ubuntu 20.04 to 22.04.

Fixes #22284

docs/production/upgrade-or-modify.md

@@ -251,7 +251,7 @@ and the latter for `server` contexts.
 ## Upgrading the operating system
 
 When you upgrade the operating system on which Zulip is installed
-(E.g. Ubuntu 18.04 Bionic to Ubuntu 20.04 Focal), you need to take
+(E.g. Ubuntu 20.04 Focal to Ubuntu 22.04 Jammy), you need to take
 some additional steps to update your Zulip installation, documented
 below.
 
@@ -259,6 +259,57 @@ The steps are largely the same for the various OS upgrades aside from
 the versions of PostgreSQL, so you should be able to adapt these
 instructions for other supported platforms.
 
+### Upgrading from Ubuntu 20.04 Focal to 22.04 Jammy
+
+1. Upgrade your server to the latest Zulip `5.x` release (at
+   least 5.3, which adds support for Ubuntu 22.04 and above).
+
+2. As the Zulip user, stop the Zulip server and run the following
+   to back up the system:
+
+   ```bash
+   supervisorctl stop all
+   /home/zulip/deployments/current/manage.py backup --output=/home/zulip/release-upgrade.backup.tar.gz
+   ```
+
+3. Switch to the root user and upgrade the operating system using the
+   OS's standard tooling. E.g. for Ubuntu, this means running
+   `do-release-upgrade` and following the prompts until it completes
+   successfully:
+
+   ```bash
+   sudo -i # Or otherwise get a root shell
+   do-release-upgrade
+   ```
+
+   When `do-release-upgrade` asks you how to upgrade configuration
+   files for services that Zulip manages like Redis, PostgreSQL,
+   nginx, and memcached, the best choice is `N` to keep the
+   currently installed version. But it's not important; the next
+   step will re-install Zulip's configuration in any case.
+
+4. As root, upgrade the database to the latest version of PostgreSQL:
+
+   ```bash
+   /home/zulip/deployments/current/scripts/setup/upgrade-postgresql
+   ```
+
+5. Next, we need to reinstall the current version of Zulip, which
+   among other things will recompile Zulip's Python module
+   dependencies for your new version of Python and rewrite Zulip's
+   full-text search indexes to work with the upgraded dictionary
+   packages:
+
+   ```bash
+   rm -rf /srv/zulip-venv-cache/*
+   /home/zulip/deployments/current/scripts/lib/upgrade-zulip-stage-2 \
+       /home/zulip/deployments/current/ --ignore-static-assets --audit-fts-indexes
+   ```
+
+   This 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 18.04 Bionic to 20.04 Focal
 
 1. Upgrade your server to the latest Zulip `3.x` or `4.x` release (at
@@ -321,6 +372,11 @@ instructions for other supported platforms.
    /home/zulip/deployments/current/scripts/setup/reindex-textual-data --force
    ```
 
+7. We recommend that you now [upgrade from Ubuntu 20.04 to
+   22.04](#upgrading-from-ubuntu-2004-focal-to-2204-jammy). Although
+   Ubuntu 20.04 is still supported at this time, Ubuntu 22.04 will be
+   supported farther into the future.
+
 ### Upgrading from Ubuntu 16.04 Xenial to 18.04 Bionic
 
 1. Upgrade your server to the latest Zulip `2.1.x` release. You can