Update gitlab/gitlab-ce Docker tag to v18.8.4 (jenkinsci#2611)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

src/main/resources/org/jenkinsci/test/acceptance/docker/fixtures/GitLabContainer/Dockerfile

@@ -1,5 +1,5 @@
 # Use the official GitLab Community Edition image as the base
-FROM gitlab/gitlab-ce:18.8.3-ce.0
+FROM gitlab/gitlab-ce:18.8.4-ce.0
 
 COPY create_user.rb /usr/bin/
 COPY gitlab.rb /etc/gitlab/