ci(woodpecker): bump alpine 3.21 -> 3.23 fuer Hugo 0.146+

PaperMod-Theme-Update verlangt Hugo >= 0.146.0, alpine:3.21
liefert nur 0.139.0. alpine:3.23 bringt 0.152.2 mit.

Triggerpfad ist Website/**, dieser Commit aendert nur .woodpecker/,
also kein Auto-Trigger — Re-Run muss im Woodpecker-UI manuell
gestartet werden.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.woodpecker/build_and_copy_website.yaml

@@ -12,7 +12,7 @@ steps:
       - date -I'seconds'
 
   - name: build_by_hugo
-    image: alpine:3.21 # hugomods/hugo # vs image: alpine >= 3.21, damit Hugo-Version >= 0.139
+    image: alpine:3.23 # PaperMod (Stand 2026-05) braucht Hugo >= 0.146; alpine:3.23 liefert 0.152.2
     commands:
       - date -I'seconds'
       - cat /etc/alpine-release