Change OS version to ubuntu-slim in CI workflow

Author: bjsowa

.github/workflows/ci.yml

@@ -23,9 +23,9 @@ jobs:
           # https://docs.ros.org/en/rolling/Releases.html
           # NOTE: Humble and Jazzy do not work on the `ros2` branch, so they are tested in their own branches.
           - ros: kilted
-            os: ubuntu-24.04
+            os: ubuntu-slim
           - ros: rolling
-            os: ubuntu-24.04
+            os: ubuntu-slim
 
     name: ROS 2 ${{ matrix.ros }} (${{ matrix.os }})
     runs-on: ${{ matrix.os }}