docs: Add documentation for bullseye support.

The support for bullseye was added in #17951
but it was not documented as bullseye was
frozen and did not have proper configuration
files, hence wasn't documented.

Since now bullseye is released as a stable
version, it's support can be documented.

(cherry picked from commit 502697d239)

docs/production/requirements.md

@@ -6,6 +6,7 @@ To run a Zulip server, you will need:
 - A supported OS:
   - Ubuntu 20.04 Focal
   - Ubuntu 18.04 Bionic
+  - Debian 11 Bullseye
   - Debian 10 Buster
 - At least 2GB RAM, and 10GB disk space
   - If you expect 100+ users: 4GB RAM, and 2 CPUs
@@ -29,10 +30,10 @@ can't support you, but
 
 #### Operating system
 
-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
+Ubuntu 20.04 Focal, 18.04 Bionic, Debian 11 Bullseye, 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

docs/production/upgrade-or-modify.md

@@ -410,6 +410,14 @@ instructions for other supported platforms.
    Bionic](#upgrading-from-ubuntu-16-04-xenial-to-18-04-bionic), so
    that you are running a supported operating system.
 
+### Upgrading from Debian Buster to Debian Bullseye
+
+We expect to have tested documentation for upgrading from Buster to
+Bullseye available soon. See [this chat.zulip.org
+thread][bullseye-discussion-thread]) for the current status of that work.
+
+[bullseye-discussion-thread]: https://chat.zulip.org/#narrow/stream/3-backend/topic/Upgrade.20to.20bullseye
+
 ### Upgrading from Debian Stretch to Debian Buster
 
 1. Upgrade your server to the latest Zulip `2.1.x` release. You can