reduce machine size (#6392)

Signed-off-by: Sally MacFarlane <macfarla.github@gmail.com>
Co-authored-by: Fabio Di Fabio <fabio.difabio@consensys.net>

.circleci/config.yml

@@ -233,7 +233,7 @@ jobs:
       - capture_test_logs
 
   acceptanceTestsCliqueBft:
-    executor: besu_executor_xl
+    executor: besu_executor_med
     steps:
       - prepare
       - attach_workspace:
@@ -242,7 +242,7 @@ jobs:
           name: AcceptanceTests (Non-Mainnet)
           no_output_timeout: 20m
           command: |
-            ./gradlew --no-daemon acceptanceTestCliqueBft
+            ./gradlew --no-daemon --max-workers=1 acceptanceTestCliqueBft
       - capture_test_results
       - capture_test_logs