message: add backend to codeowners
diff: /plugins/search @backstage/reviewers @backstage/techdocs-core
  /plugins/search-* @backstage/reviewers @backstage/techdocs-core
  /plugins/stack-overflow @backstage/reviewers @backstage/techdocs-core
  +/plugins/stack-overflow-backend @backstage/reviewers @backstage/techdocs-core
  /plugins/sonarqube @backstage/reviewers @backstage/sda-se-reviewers
  /plugins/techdocs @backstage/reviewers @backstage/techdocs-core
  /plugins/techdocs-* @backstage/reviewers @backstage/techdocs-core
  