Update logstash-releases.json (elastic#13418)

This update incorporates:
  * Latest logstash release on `7.15` branch
  * Release of 8.0.0-beta1
  * Update of 8.x snapshot to 8.1.0-SNAPSHOT

This follows the precedence of elastic#10627 to update snapshot release of 8.x to 8.1 following pre-GA releases
of major branch.

ci/logstash_releases.json

@@ -2,10 +2,11 @@
   "releases": {
     "5.x": "5.6.16",
     "6.x": "6.8.20",
-    "7.x": "7.15.1"
+    "7.x": "7.15.2",
+    "8.x": "8.0.0-beta1"
   },
   "snapshots": {
     "7.x": "7.16.0-SNAPSHOT",
-    "8.x": "8.0.0-SNAPSHOT"
+    "8.x": "8.1.0-SNAPSHOT"
   }
-}
+}