Add "Searchable Snapshots" to changelog validation schema (elastic#129180)

We created a new ":Distributed Indexing/Searchable Snapshots" label recently on Github, so I think it makes sense to also have a "Searchable Snapshots" label in the changelog. It also makes sense since there is automatic changelog generation based on the pull request label.

Author: tlrx

build-tools-internal/src/main/resources/changelog-schema.json

@@ -86,6 +86,7 @@
             "Rollup",
             "SQL",
             "Search",
+            "Searchable Snapshots",
             "Security",
             "Snapshot/Restore",
             "Stats",