Remove Docker Buildx setup step from server CI

The Buildx builder is used by default since Moby 23.0.0, bundled into
Docker Desktop 4.19

.github/workflows/server-cd.yml

@@ -55,17 +55,14 @@ jobs:
           name: server
           path: server/build
 
-      - name: set up Docker Buildx
-        uses: docker/setup-buildx-action@v2
-
       - name: log in to DockerHub
         uses: docker/login-action@v2
         with:
           username: ${{ secrets.DOCKER_USERNAME }}
           password: ${{ secrets.DOCKER_PASSWORD }}
 
       - name: build and publish server Docker image
-        uses: docker/build-push-action@v3
+        uses: docker/build-push-action@v4
         with:
           context: server
           file: server/docker/Dockerfile