add output on how many got deleted

ATM the number of deleted items are only logged:

```
Run actions/delete-package-versions@v5
Total versions deleted till now: 2
```

It would be great to have these values also as an output, so that we can put it on the job summary, e.g:

```
   echo:  ${{ steps.purge.outputs.deletion_count }} >> $GITHUB_STEP_SUMMARY
```