Actions: Update upload-artifact to v6

Author: simmsa

.github/workflows/main.yml

@@ -108,7 +108,7 @@ jobs:
           pytest mhkit/tests/wave/io/test_cdip.py
 
       - name: Upload data as artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: data
           path: ~/.cache/mhkit
@@ -151,7 +151,7 @@ jobs:
           pytest mhkit/tests/wave/io/hindcast/test_hindcast.py
 
       - name: Upload Wave Hindcast data as artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: wave-hindcast-data
           path: ~/.cache/mhkit
@@ -194,7 +194,7 @@ jobs:
           pytest mhkit/tests/wave/io/hindcast/test_wind_toolkit.py
 
       - name: Upload Wind Hindcast data as artifact
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: wind-hindcast-data
           path: ~/.cache/mhkit