Task A-6: Switching to step outputs instead of GITHUB_ENV

Refer to details in cleanup issue:
Task A-6: e-mission/e-mission-docs#1082 (comment)

.github/workflows/image_build_push.yml

@@ -23,7 +23,7 @@ jobs:
 
     - name: Get current date # get the date of the build
       id: date
-      run: echo "::set-output name=date::$(date +'%Y-%m-%d--%M-%S')"
+      run: echo "date=$(date +'%Y-%m-%d--%M-%S')" >> "$GITHUB_OUTPUT"
 
     - name: Run a one-line script
       run: echo running in repo ${GITHUB_REPOSITORY#*/} branch ${GITHUB_REF##*/} on ${{ steps.date.outputs.date }}