Fix rebase failure alert variant

What does this MR do and why?

There is no error variant for alerts, so passing that yields an unstyled alert banner. This MR fixes this functionality by passing the danger prop.

References

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• ✂️ UX Paper Cuts 17.10 → Accessibility & AI Age... (#512691 - closed)

MR acceptance checklist

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Before	After

How to set up and validate locally

1. Check out this branch
2. Visit an MR in your GDK that is multiple commits behind the tip of its repo's main branch
3. apply this patch so that an attempted rebase will fail:

diff --git a/app/assets/javascripts/vue_merge_request_widget/components/states/ready_to_merge.vue b/app/assets/javascripts/vue_merge_request_widget/components/states/ready_to_merge.vue
index 3ff2488d8787..2a5b2cdd6021 100644
--- a/app/assets/javascripts/vue_merge_request_widget/components/states/ready_to_merge.vue
+++ b/app/assets/javascripts/vue_merge_request_widget/components/states/ready_to_merge.vue
@@ -565,7 +565,7 @@ export default {
           `${this.mr.iid}`,
           'rebase',
         );
-        await axios.post(rebasePath);
+        await axios.post();
 
         createAlert({
           message: sprintf(this.$options.i18n.scheduledRebase, {

4. Click the Rebase source branch button on the MR.
5. You should see a correctly styled danger-variant alert banner at the top of the page.