bump gitlab plugin version to 1.6.0 (#22057)

Co-authored-by: Mattermost Build <build@mattermost.com>

Makefile

@@ -154,7 +154,7 @@ PLUGIN_PACKAGES += mattermost-plugin-channel-export-v1.0.0
 PLUGIN_PACKAGES += mattermost-plugin-confluence-v1.3.0
 PLUGIN_PACKAGES += mattermost-plugin-custom-attributes-v1.3.1
 PLUGIN_PACKAGES += mattermost-plugin-github-v2.1.4
-PLUGIN_PACKAGES += mattermost-plugin-gitlab-v1.5.2
+PLUGIN_PACKAGES += mattermost-plugin-gitlab-v1.6.0
 PLUGIN_PACKAGES += mattermost-plugin-playbooks-v1.35.0
 PLUGIN_PACKAGES += mattermost-plugin-jenkins-v1.1.0
 PLUGIN_PACKAGES += mattermost-plugin-jira-v3.2.2