ci: correctly handle multiple open milestones

[nikku]

Proposed Changes

As observed in this action run our automatic assignment utility insufficiently handles multiple open development milestones. These can be open in case we push things to a future milestone.

The following query properly returns only the first (supposed to be active) development milestone:

gh api -H "Accept: application/vnd.github.v3+json" \
    /repos/camunda/camunda-modeler/milestones \
    --jq '[.[] | select(.title | startswith("M")) | .number ][0]'

Checklist

To ensure you provided everything we need to look at your PR:

• Brief textual description of the changes present
• Visual demo attached
• Steps to try out present, i.e. using the @bpmn-io/sr tool
• Related issue linked via Closes {LINK_TO_ISSUE} or Related to {LINK_TO_ISSUE}