Specify version of comment acttion

Instead of just specifying the main branch, this pull request specifies
the latest major version of thollander/actions-comment-pull-request to
use in GitHub Actions. We don't know how this action changes and
tracking the default branch may cause this to break at any time. We can
always update using Dependabot.

.github/workflows/update-test.yml

@@ -92,9 +92,8 @@ jobs:
         git push origin gh-pages
 
     - name: Add comment with deployment location
-      uses: thollander/actions-comment-pull-request@main
+      uses: thollander/actions-comment-pull-request@v2
       with:
-        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         message: >
           This pull request is deployed at
           [test.editor.opencast.org/${{ steps.build-path.outputs.build }}