Add script for creating milestones on all Gazebo libraries

[azeey]

Tools and data to create milestones on all Gazebo
libraries as it is not currently possible to share a milestone between different
Github repositories. Instead, we create a milestone with the same name on all
the repos and use queries like
milestone:"Jetty Release"
to filter pull request/issues. This is typically done during preparation for the
release of a collection (e.g. Gazebo Jetty).

AI Note: Generated using Gemini 2.5 Pro

[j-rivero]

The 68-73 lines are a bit over-complex to my eyes probably with some bugs like the use of retval instead of $retval in line 71. Can not try this easily but I would write something like:

--- a/release-collection/milestones/create_milestones.bash
+++ b/release-collection/milestones/create_milestones.bash
@@ -65,10 +65,11 @@ ros_gz
 
 for repo in $repos; do
   echo "Creating milestone on gazebosim/$repo"
-  result=$(cat $1 | gh api  --jq ".html_url" --method POST repos/gazebosim/$repo/milestones --input - 2>&1)
-  retval=$?
-  echo "  url: $result"
-  if [[ retval -ne 0 ]]; then
+  result=$(gh api --jq ".html_url" --method POST "repos/gazebosim/$repo/milestones" --input "$1" 2>&1)
+  if [[ $? -eq 0 ]]; then
+    echo "  url: $result"
+  else
+    echo "Error: $result" >&2
     exit 1
   fi
 done