migrated removed GH action "::set-env" command

to the new approach as documented here:
https://docs.github.com/en/free-pro-team@latest/actions/reference/workflow-commands-for-github-actions#environment-files

Author: thjaeckle

.github/workflows/push-dockerhub.yml

@@ -17,10 +17,10 @@ jobs:
     - name: Branch name
       id: branch_name
       run: |
-        echo "::set-env name=IMAGE_TAG::${GITHUB_REF#refs/tags/}"
-        echo "::set-env name=IMAGE_MINOR_TAG::$(echo ${GITHUB_REF#refs/tags/} | cut -d. -f-2)"
-        echo "::set-env name=IMAGE_MAJOR_TAG::$(echo ${GITHUB_REF#refs/tags/} | cut -d. -f-1)"
-        echo "::set-env name=MILESTONE_OR_RC_SUFFIX::$(echo ${GITHUB_REF#refs/tags/} | cut -d- -f2)"
+        echo "IMAGE_TAG=${GITHUB_REF#refs/tags/}" >> $GITHUB_ENV
+        echo "IMAGE_MINOR_TAG=$(echo ${GITHUB_REF#refs/tags/} | cut -d. -f-2)" >> $GITHUB_ENV
+        echo "IMAGE_MAJOR_TAG=$(echo ${GITHUB_REF#refs/tags/} | cut -d. -f-1)" >> $GITHUB_ENV
+        echo "MILESTONE_OR_RC_SUFFIX=$(echo ${GITHUB_REF#refs/tags/} | cut -d- -f2)" >> $GITHUB_ENV
     - name: Building Docker images for tag
       run: |
         echo $IMAGE_TAG