Update OpenBSD versions, fetch from CDN

7.6 became unsupported, snapshots switched from 7.7 to 7.8
and https://openbsd.org/78.html got released.

Make snapshots the first/default entry, their use is encouraged.

ftp.o.o is the primary distribution source,
pick a mirror from https://www.openbsd.org/ftp.html instead.

roles/netbootxyz/defaults/main.yml

@@ -401,18 +401,18 @@ releases:
     base_dir: pub/OpenBSD
     enabled: true
     menu: bsd
-    mirror: http://ftp.openbsd.org
+    mirror: http://cdn.openbsd.org
     name: OpenBSD
     versions:
+    - code_name: snapshots
+      image_ver: '78'
+      name: 7.8 Latest Snapshot
+    - code_name: '7.8'
+      image_ver: '78'
+      name: '7.8'
     - code_name: '7.7'
       image_ver: '77'
       name: '7.7'
-    - code_name: '7.6'
-      image_ver: '76'
-      name: '7.6'
-    - code_name: snapshots
-      image_ver: '77'
-      name: 7.7 Latest Snapshot
   opensuse:
     base_dir: distribution/leap
     enabled: true