authentik: 2026.2.3 -> 2026.5.6, the mandatory intermediate step (#0051)

Authentik's documentation forbids skipping major releases and requires the
latest minor of each before moving on, so 2026.5.6 comes before 2026.8.0 rather
than being an optional stop.

It is also where the benefit is. Measured on the images rather than assumed:
2026.2.3 carries 27 critical and 477 high findings, 2026.5.6 carries 5 and 92.
This one step removes every critical finding that can be removed here; what
remains is perl-base and libxml2, neither of which has a fix. The second step
adds high findings only.

Both breaking changes of this release were checked against our state and do not
apply: the deprecated Postgres connection options are not set anywhere, and
there are no outposts whose version would have to match.

A manual backup was taken immediately before this, because the nightly one is
hours old and the way back is a database restore rather than a version rollback.

apps/authentik/authentik.yaml

@@ -8,7 +8,15 @@ spec:
   chart:
     spec:
       chart: authentik
-      version: "2026.2.3"
+      # 2026.5.6 ist die LETZTE 2026.5 - Authentik verlangt "always upgrade to the
+      # latest minor version before upgrading to the next major version", und
+      # Fassungsreihen duerfen nicht uebersprungen werden. Naechster Schritt nach
+      # gruener Pruefung: 2026.8.0 (management #0051, Gate 2).
+      #
+      # Am Zielimage gemessen, nicht geschaetzt: 2026.2.3 traegt 27 CRITICAL und
+      # 477 HIGH, 2026.5.6 nur noch 5 und 92. Die Zwischenstufe bringt damit den
+      # gesamten CRITICAL-Gewinn; 2026.8.0 liefert danach nur noch HIGH.
+      version: "2026.5.6"
       sourceRef:
         kind: HelmRepository
         name: goauthentik