Replace deprecated command with environment file (#440)

.github/workflows/ci.yml

@@ -65,7 +65,7 @@ jobs:
         name: Extract current version
         run: |
           version=$(cat gradle.properties | grep "version=" | awk -F'=' '{print $2}')
-          echo "::set-output name=project_version::$version"
+          echo "project_version=$version" >> $GITHUB_OUTPUT
 
   perform_release:
     name: Perform Release

.github/workflows/prepare-release.yml

@@ -36,7 +36,7 @@ jobs:
         name: Check for open issues
         run: |
           echo "TODO actually check for open issues"
-          echo "::set-output name=is_open_issues::false"
+          echo "is_open_issues=false" >> $GITHUB_OUTPUT
 
       - id: validate-release-state
         name: Validate release state