Update instructions for installing etcd3gw, for OpenStack (#2610)

Author: nelljerram

calico/getting-started/openstack/installation/redhat.mdx

@@ -58,7 +58,7 @@ These steps are detailed in this section.
     EOF
     ```
 
-1.  Install version 1.0.1 of the `etcd3gw` Python package.  This is
+1.  Install the `etcd3gw` Python package.  This is
     needed by $[prodname]'s OpenStack driver and DHCP agent.
 
     ```

calico/getting-started/openstack/installation/ubuntu.mdx

@@ -58,9 +58,14 @@ These steps are detailed in this section.
     apt-get update
     ```
 
-1.  Install version 1.0.1 of the `etcd3gw` Python package.  This is
-    needed by $[prodname]'s OpenStack driver and DHCP agent.
-
+1.  Install the `etcd3gw` Python package.  This is needed by $[prodname]'s
+    OpenStack driver and DHCP agent.  On Ubuntu 24.04 (Noble) and later, use the
+    version included in the Ubuntu archive:
+    ```bash
+    apt-get install -y python3-etcd3gw
+    ```
+    On Ubuntu 22.04 (Jammy) or earlier, etcd3gw is either too old in the Ubuntu
+    archive, or not available at all, so instead install from PyPI:
     ```
     apt-get install -y python3-pip
     pip3 install etcd3gw==2.4.0