Fix dev vulnerabilities seeder

On this [MR](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/25609) the `vulnerability#close_by` was renamed to `vulnerability#dismissed_by`.
This commit updates the name on the vulnerability seeder.

changelogs/unreleased/kassio-fix-dev-seed.yml

+---
+title: Fix dev vulnerabilities seeder
+merge_request: 26169
+author:
+type: fixed

ee/db/fixtures/development/20_vulnerabilities.rb

@@ -45,7 +45,7 @@ class Gitlab::Seeder::Vulnerabilities
     when :resolved
       vulnerability.resolved_by = author
     when :dismissed
-      vulnerability.closed_by = author
+      vulnerability.dismissed_by = author
     end
 
     vulnerability.tap(&:save!)