Reduce code duplication

Use reusable workflows

.gitea/workflows/publish.yaml

@@ -15,7 +15,7 @@ jobs:
 
     steps:
       - name: Download build artifact
-        uses: actions/download-artifact@v3
+        uses: actions/download-artifact@v4
         with:
           name: polycule.png
           path: ./

.gitea/workflows/render.yaml

@@ -25,7 +25,7 @@ jobs:
       - name: Render diagram
         run: java -jar plantuml-${{env.PLANTUML_VERSION}}.jar polycule.puml
       - name: Upload build artifact
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: polycule.png
           path: polycule.png