gitlab: Update chart to 10.2.6 and disable bundled Prometheus.

The Prometheus deployment never ran (its PVC has no storage class), and
chart 10.2 changed its immutable selector, which blocked the upgrade.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Change-Id: Ic701fef0cbbdd6784a98abc1ff76754fb8d1c683
diff --git a/gitlab-system/gitlab.yaml b/gitlab-system/gitlab.yaml
index c0f4dba..f9d068f 100644
--- a/gitlab-system/gitlab.yaml
+++ b/gitlab-system/gitlab.yaml
@@ -5,7 +5,7 @@
   namespace: gitlab-system
 spec:
   chart:
-    version: "10.1.9"
+    version: "10.2.6"
     values:
       global:
         appConfig:
@@ -194,6 +194,9 @@
       postgresql:
         install: false
 
+      prometheus:
+        install: false
+
       redis:
         install: false