CI: Update actions/upload-artifact to use v4

v3 is deprecated since 2024 and CI builds automatically fail as a result. This is required to restore automated build functionality.

Author: JoshSanch

.github/workflows/build.yml

@@ -16,7 +16,7 @@ jobs:
           source_file: src/main.typ
 
       - name: Upload PDF file
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: PDF
           path: src/main.pdf