CI: Drop iron from rosdistro matrix

Iron is end-of-life, so its rosdistro keys aren't resolved any more.

Author: mintar

.github/workflows/github-actions.yml

@@ -13,7 +13,7 @@ jobs:
   build:
     strategy:
       matrix:
-        rosdistro: [humble, iron, jazzy]
+        rosdistro: [humble, jazzy]
       fail-fast: false
     runs-on: ubuntu-latest
     container: