reset downstream branch

ci.jsonnet

@@ -246,7 +246,7 @@ local contains(str, needle) = std.findSubstr(needle, str) != [];
     # Downstream Graal branch to test against. If you change this value to anything but
     # "master", you must create an ol-jira issue to change it back to master once the
     # next JVMCI release has been made. Add the issue id as a comment here.
-    local downstream_branch = "labsjdk/automation-1-26-2024-4799",
+    local downstream_branch = "release/graal-vm/24.0",
 
     local clone_graal(defs) = {
         # Checkout the graal-enterprise repo to the "_gate" version of the