Fix deprecated upload-artifact action

Update actions/upload-artifact from v3 to v4 in GitHub workflows
to resolve deprecation warning and CI failure.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: payne0420

.github/workflows/build.yml

@@ -46,7 +46,7 @@ jobs:
           "Build/"
     
     - name: Upload build artifacts
-      uses: actions/upload-artifact@v3
+      uses: actions/upload-artifact@v4
       with:
         name: iTermGUI-${{ github.sha }}
         path: Build/iTermGUI.app

.github/workflows/nightly.yml

@@ -45,7 +45,7 @@ jobs:
           "Build/"
     
     - name: Upload nightly build
-      uses: actions/upload-artifact@v3
+      uses: actions/upload-artifact@v4
       with:
         name: iTermGUI-nightly-${{ github.run_number }}
         path: |