Revert resource_class to default medium

as the build resource usage seldom maxes out 4 cores
nor uses more than 2GB RAM

Author: ruffsl

.circleci/config.yml

@@ -476,7 +476,7 @@ executors:
   release_exec:
     docker:
       - image: ghcr.io/ros-planning/navigation2:main
-    resource_class: large
+    resource_class: medium
     working_directory: /opt/overlay_ws
     environment:
       <<: *common_environment

.circleci/defaults.yaml

@@ -6,7 +6,7 @@ _common: &common
 "build":
   <<: *common
   "executor": "parallel"
-  "parallel-workers": 4
+  "parallel-workers": 2
   "symlink-install": true
 "test":
   <<: *common