Fix alert that always shows missing string

- the vulnerability.default_branch_name property is been renamed
  vulnerability.project_default_branch
- update tests

ee/app/assets/javascripts/vulnerabilities/components/header.vue

@@ -146,7 +146,7 @@ export default {
   <div>
     <resolution-alert
       v-if="showResolutionAlert"
-      :default-branch-name="vulnerability.default_branch_name"
+      :default-branch-name="vulnerability.project_default_branch"
     />
     <div class="detail-page-header">
       <div class="detail-page-header-body align-items-center">

ee/spec/frontend/vulnerabilities/header_spec.js

@@ -208,7 +208,7 @@ describe('Vulnerability Header', () => {
     beforeEach(() => {
       createWrapper({
         resolved_on_default_branch: true,
-        default_branch_name: branchName,
+        project_default_branch: branchName,
       });
     });