Update deprecated artefact-upload version (#126)

https://github.blog/changelog/2024-04-16-deprecation-notice-v3-of-the-artifact-actions/

Author: fschledorn

.github/workflows/build.yml

@@ -14,7 +14,7 @@ jobs:
             apk add g++
             apk add zip
             make script zip
-      - uses: actions/upload-artifact@v2
+      - uses: actions/upload-artifact@v4
         with:
           name: PDF
           path: vorkurs.pdf
@@ -25,4 +25,4 @@ jobs:
           files: |
             vorkurs.pdf
             vorkurs.zip
-      
+