Fix Imported badge bg color conflict over sticky header

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What does this MR do and why?

This MR fixes #457213.

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

Screenshots or screen recordings

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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,