WIP: Update upgrade docs

docs/source/installation/upgrade.rst

@@ -26,13 +26,14 @@ Now switch to the *indico* user and activate the virtualenv:
     su - indico
     source ~/.venv/bin/activate
 
-If you are on CentOS, update your PATH to avoid errors in case the new
+If you use Alma/Rocky, update your PATH to avoid errors in case the new
 Indico version needs to install an updated version of the PostgreSQL client
-library (psycopg2):
+library (psycopg2). If you use a different Postgres version, you need to
+adapt the command accordingly:
 
 .. code-block:: shell
 
-    export PATH="$PATH:/usr/pgsql-13/bin"
+    export PATH="$PATH:/usr/pgsql-16/bin"
 
 You are now ready to install the latest version of Indico:
 
@@ -86,6 +87,27 @@ Also start the Celery worker again (once again, as *root*):
     systemctl start indico-celery.service
 
 
+Upgrading from 3.x to 3.3
+-------------------------
+
+When updating to version 3.3 you need to perform some extra steps since the required
+Python version has been raised from 3.9 to 3.12.
+
+.. important::
+
+    If you are still using **CentOS 7** (or 8), you are required to **update your operating
+    system** to version **9** - your system is not only too old, but also going end-of-life
+    in the middle of 2024. Since CentOS no longer exists in its original form, we recommend
+    using **AlmaLinux 9** or **Rocky Linux 9**. You can find guides on the internet on how
+    to do such an upgrade in-place (we never tried this!), but it is almost certainly easier
+    to just use a new VM and migrate your Indico installations to that one. Please also read
+    the "2.x to 3.x" guide below which explains how to migrate an Indico instance to a new system.
+
+The general "between 3.x versions" upgrade guide above still applies, you just need to
+perform some additional steps.
+
+TODO...
+
 
 Upgrading from 2.x to 3.x
 -------------------------