Fix Imported badge bg color conflict over sticky header

• Please check this box if this contribution uses AI-generated content as outlined in the GitLab DCO & CLA

What does this MR do and why?

This MR fixes #457213 (closed).

Thanks to the suggestion in discussion #457213 (comment 1875327035) by @clavimoniere.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Same as specified in !149876 (merged)

Go to Project > Plan > Issues and open any issue. For example, http://127.0.0.1:3000/gitlab-org/gitlab-test/-/issues/3. Apply the following diff to show the badge:

diff --git a/app/assets/javascripts/issues/show/components/app.vue b/app/assets/javascripts/issues/show/components/app.vue
index c4f254880852..28cbe41c8114 100644
--- a/app/assets/javascripts/issues/show/components/app.vue
+++ b/app/assets/javascripts/issues/show/components/app.vue
@@ -152,7 +152,7 @@ export default {
     isImported: {
       type: Boolean,
       required: false,
-      default: false,
+      default: true,
     },
     isLocked: {
       type: Boolean,