[CRCR] update default zombie timeout to 6 hours (#8292)

24 hours is too long for a downstream job to time out. GitHub's default
timeout value is 6 hours:
https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax#jobsjob_idtimeout-minutes

Let's align with GitHub.

Author: can-gaa-hou

aws/lambda/cross_repo_ci_relay/utils/config.py

@@ -155,9 +155,9 @@ def from_env(cls) -> "RelayConfig":
         rate_limit_per_min = _check_if_positive_int("RATE_LIMIT_PER_MIN", "100")
 
         # Maximum duration an in-progress job is expected to run before being
-        # considered abandoned (zombie).  Default 24 hours (86400 s).
+        # considered abandoned (zombie).  Default 6 hours (21600 s).
         zombie_timeout_seconds = _check_if_positive_int(
-            "ZOMBIE_TIMEOUT_SECONDS", "86400"
+            "ZOMBIE_TIMEOUT_SECONDS", "21600"
         )
 
         max_cleanup_workers = _check_if_positive_int("MAX_CLEANUP_WORKERS", "16")