add livecodebench v6 (#712)

Author: Cppowboy

src/lighteval/tasks/extended/lcb/main.py

@@ -120,12 +120,14 @@ def codegen_metric(predictions: list[str], formatted_doc: Doc, **kwargs) -> floa
     "release_v3",
     "release_v4",
     "release_v5",
+    "release_v6",
     "release_latest",
     "v1",
     "v2",
     "v3",
     "v4",
     "v5",
+    "v6",
     "v1_v2",
     "v1_v3",
     "v1_v4",