CI: actions/upload-artifact skip version

fixing complains:
```
Node.js 16 actions are deprecated. Please update the
following actions to use Node.js 20: actions/cache@v3,
actions/upload-artifact@v3. For more information see:
https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.
```

Use the main branch instead of hard-coded the version to eliminate
the needs to such small/technical fixes in future. Taken from:
<https://stackoverflow.com/questions/73256369/using-the-latest-version-of-a-github-action#73259281>

.github/workflows/arm-build.yml

@@ -85,7 +85,7 @@ jobs:
         sudo .github/scripts/replace-asset.sh GITHUB_REPOSITORY=$GITHUB_REPOSITORY GITHUB_TOKEN=${{ secrets.GITHUB_TOKEN }} continuous ~/chroot/UltraGrid/UltraGrid-latest-${{ matrix.arch }}.AppImage application/x-appimage Linux%20${{ matrix.arch }}%20build
     - name: Upload Build
       if: steps.upload-release-asset.conclusion == 'skipped'
-      uses: actions/upload-artifact@v3
+      uses: actions/upload-artifact@main
       with:
         name: UltraGrid CI ${{ matrix.arch }} build
         path: '~/chroot/UltraGrid/UltraGrid-latest-${{ matrix.arch }}.AppImage'

.github/workflows/ccpp.yml

@@ -137,7 +137,7 @@ jobs:
         .github/scripts/replace-asset.sh $TAG UltraGrid-$VERSION-x86_64.AppImage application/x-appimage Linux%20build
     - name: Upload Build
       if: steps.upload-release.conclusion == 'skipped'
-      uses: actions/upload-artifact@v3
+      uses: actions/upload-artifact@main
       with:
         name: UltraGrid-Linux
         path: UltraGrid-${{ env.VERSION }}-x86_64.AppImage
@@ -228,7 +228,7 @@ jobs:
         .github/scripts/replace-asset.sh $TAG UltraGrid-$VERSION.dmg application/x-apple-diskimage macOS%20build
     - name: Upload Build
       if: steps.upload-release.conclusion == 'skipped'
-      uses: actions/upload-artifact@v3
+      uses: actions/upload-artifact@main
       with:
         name: UltraGrid-macOS
         path: UltraGrid-${{ env.VERSION }}.dmg
@@ -351,7 +351,7 @@ jobs:
         $GITHUB_WORKSPACE/.github/scripts/replace-asset.sh $TAG UltraGrid-$VERSION-win64.zip application/zip Windows%20build
     - name: Upload Build
       if: steps.upload-release.conclusion == 'skipped'
-      uses: actions/upload-artifact@v3
+      uses: actions/upload-artifact@main
       with:
         name: UltraGrid-Windows
         path: build