Fix closing answered issues by setting the stale-issue-message attrib…

…ute.

Despite what the stale action docs say in https://github.com/actions/stale#stale-issue-message:

> You can skip the comment sending by omitting the option or by passing an empty string.

we are seeing an error that prevents issues from getting marked as closing (https://github.com/jupyterlab/jupyterlab/runs/2994705101?check_suite_focus=true#step:2:3995):

> [#5869] Skipping this issue because it should be marked as stale based on the option days-before-stale (​https://github.com/actions/stale#days-before-stale​) (30) but the option stale-issue-message (​https://github.com/actions/stale#stale-issue-message​) is not set

.github/workflows/answered.yml

@@ -13,8 +13,8 @@ jobs:
     steps:
       - uses: actions/stale@v3
         with:
-          skip-stale-issue-message: true
           days-before-stale: 30
           days-before-close: 7
           stale-issue-label: 'status:Closing as Answered'
+          stale-issue-message: ''
           only-issue-labels: 'status:Answered'