data/keyboards/de*.yaml: Change äÄ-label to Ää

A big letter after a small letter looks incorrect on a keyboard,
because that is not normal German grammar and a keyboard is for
inputting text.
The key is also on the left side of the layout,
which makes äÄ not fit the visual flow.

Part-of: <https://gitlab.gnome.org/World/Phosh/squeekboard/-/merge_requests/611>

data/keyboards/de.yaml

@@ -71,7 +71,7 @@ buttons:
                 lock_view: "eschars"
                 unlock_view: "base"
         outline: "altline"
-        label: "äÄ"
+        label: "Ää"
     space:
         outline: "spaceline"
         label: " "