Update logstash_releases.json for new branching strategy (#16585)

See naming rational in logstash-plugins/.ci#63 (comment)
After 8.16 is GA, the "releases" entry should become:

```json
  "releases": {
    "7.current": "7.17.24",
    "8.current": "8.16.0",
    "8.previous": "8.15.3"
  },
```

For snapshots we'll also test against "main", "8.next", and "8.future". The labels are:

- `main`: main branch
-  `8.future`: the future 8.x release, i.e. current version of the 8.x branch
- `8.next`: the short lived period between a minor's FF - when the new branch is cut from 8.x - and GA
- `8.current`: the most recent 8.x release
- `8.previous`: the previous, but still supported, 8.x release

ci/logstash_releases.json

@@ -1,14 +1,15 @@
 {
   "releases": {
-    "5.x": "5.6.16",
-    "6.x": "6.8.23",
-    "7.x": "7.17.25",
-    "8.x": "8.15.3"
+    "7.current": "7.17.25",
+    "8.previous": "8.15.4",
+    "8.current": "8.16.0"
   },
   "snapshots": {
-    "7.x": "7.17.26-SNAPSHOT",
-    "8.15": "8.15.4-SNAPSHOT",
-    "8.x": "8.16.0-SNAPSHOT",
+    "7.current": "7.17.26-SNAPSHOT",
+    "8.previous": "8.15.5-SNAPSHOT",
+    "8.current": "8.16.1-SNAPSHOT",
+    "8.next": null,
+    "8.future": "8.17.0-SNAPSHOT",
     "main": "9.0.0-SNAPSHOT"
   }
 }