fix: Add missing screenshot to Gardener upgrade How-To

A screenshot from the Gardener upgrade How-To guide is
missing, so we make sure it's there. While at it, we refresh
all screenshots and introduce slight modifications to prose
due to minor UI changes.

docs/howto/kubernetes/gardener/rolling-upgrades.md

@@ -21,25 +21,25 @@ In the left-hand vertical pane of the {{gui}}, select Containers →
 [{{k8s_management_service}}](https://{{gui_domain}}/containers/gardener).
 In the central pane, you will see all your {{k8s_management_service}}
 clusters, each on its own row. If a Kubernetes upgrade is available, in
-the *Versions* column you will notice a triangular marker together with
-the new Kubernetes version. On the other hand, if there is a new machine
-image version for the cluster nodes, then in the *Workers* column you
-will notice just a triangular marker and no version number.
+the *Version* column you will notice a blue :fontawesome-solid-circle-arrow-up: icon, together with
+the new Kubernetes version. Additionally, if there is a new machine
+image version for the cluster nodes, then in the *Worker Groups* column you
+will also notice a blue :fontawesome-solid-circle-arrow-up: icon but no version number.
 
 ![Garden Linux and Kubernetes upgrades available](assets/rollupgr-01.png)
 
 To get more information about the new machine image available, click on
-the cluster row to bring its details into view, then select the *Worker
+the cluster row to bring its details up into view, then select the *Worker
 Groups* tab. There, you will see the version of the new machine image.
 To start the upgrade process, click any of the two
-:fontawesome-solid-arrows-rotate: icons.
+:fontawesome-solid-arrow-up: icons.
 
 ![About to start Garden Linux upgrade](assets/rollupgr-02.png)
 
 The upgrade process will start immediately and will take some time to
-complete. After it is done, there will be no triangular marker in the
-*Workers* column, and the new image version will be displayed in the
-*Worker groups* tab. Our example still has a triangular marker in the
+complete. A little before and after it is done, there will be no icon in the
+*Worker Groups* column, and the new image version will be displayed in the
+*Image* row of the *Worker Groups* tab. Our example still has an icon in the
 *Version* column. That is because we have not upgraded Kubernetes yet.
 
 ![Garden Linux upgraded](assets/rollupgr-03.png)
@@ -62,7 +62,16 @@ to.  When ready, click the blue *Upgrade* button to start the process.
 
 ![About to start a Kubernetes upgrade](assets/rollupgr-05.png)
 
+You will realize that the upgrade doesn't start right away. Instead, you
+are informed that the cluster is indeed ready for it, but there are some
+notes you might want to read up on before the whole process begins. If
+you do, copy and paste the provided URL into a separate browser tab or
+window. In any case, to actually get the upgrade going, click the
+*Upgrade* button which is now green.
+
+![Upgrade notes are available](assets/rollupgr-06.png)
+
 After some minutes, the upgrade will be over. The new Kubernetes version
 will be visible in the *Version* column of the cluster row.
 
-![Kubernetes upgraded](assets/rollupgr-06.png)
+![Kubernetes successfully upgraded](assets/rollupgr-07.png)