Update GitVersion action to use version 6.x

Author: tsutomi

.github/workflows/cicd.yml

@@ -28,7 +28,7 @@ jobs:
       - name: Install GitVersion
         uses: gittools/actions/gitversion/setup@v4
         with:
-          versionSpec: '5.x'
+          versionSpec: '6.x'
 
       - name: Determine Version
         id: gitversion