fix: using a specific version for add-to-project (#800)

Using a specific version for add-to-project as recommended by the [author of the tool](actions/add-to-project#239).
*By submitting this pull request, I confirm that my contribution is made under the terms of the 
[Monada Contribution License](https://docs.winglang.io/terms-and-policies/contribution-license.html)*.

.github/workflows/issue-backlogger.yml

@@ -8,7 +8,7 @@ jobs:
     name: "Add issue to backlog"
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/add-to-project@RELEASE_VERSION
+      - uses: actions/add-to-project@v0.4.0
         with:
           project-url: https://github.com/orgs/winglang/projects/2
           github-token: ${{ secrets.PROJEN_GITHUB_TOKEN }}