CI: Setup dotnet 6 in docgen (DevExpress#6977)

Author: alexslavr

.github/workflows/docgen.yml

@@ -10,15 +10,15 @@ jobs:
 
     steps:
         - name: Checkout Docs
-          uses: actions/checkout@v3
+          uses: actions/checkout@v4
           with:
             path: documentation
             token: ${{ secrets.DXROBOT_PAT }}
             ssh-key: ${{ secrets.DXROBOT_SK }}
             ref: ${{ github.ref_name }}
 
         - name: Checkout DevExtreme
-          uses: actions/checkout@v3
+          uses: actions/checkout@v4
           with:
             path: devextreme
             repository: devexpress/devextreme
@@ -29,6 +29,11 @@ jobs:
             version: 9
             run_install: false
 
+        - name: Set up .NET
+          uses: actions/setup-dotnet@v4
+          with:
+            dotnet-version: 6.0.x
+
         - name: NPM - devextreme
           working-directory: devextreme
           run: pnpm install