Add gardenlinux 1877.6 (#984)

Signed-off-by: Christian Berendt <berendt@osism.tech>

Author: berendt

etc/images/gardenlinux.yml

@@ -34,3 +34,8 @@ images:
         mirror_url: https://nbg1.your-objectstorage.com/osism/openstack-images/gardenlinux/1877.5/openstack-gardener_prod-amd64-1877.5-7a907e4e.qcow2
         checksum: "sha256:1ad90c202e360f0893525a081dd381dcdfbb42242da2c249bad3284fef6030a7"
         build_date: 2025-10-17
+      - version: '1877.6'
+        url: https://github.com/gardenlinux/gardenlinux/releases/download/1877.6/openstack-gardener_prod-amd64-1877.6-48d68617.tar.xz
+        mirror_url: https://nbg1.your-objectstorage.com/osism/openstack-images/gardenlinux/1877.6/openstack-gardener_prod-amd64-1877.6-48d68617.qcow2
+        checksum: "sha256:1ad90c202e360f0893525a081dd381dcdfbb42242da2c249bad3284fef6030a7"
+        build_date: 2025-11-04