Remove stale GitLab toolbox istio shutdown value

General MR

Summary

• Removes the stale GitLab toolbox backup cron istioShutdown value that referenced the Istio sidecar /quitquitquit endpoint.
• Updates maintenance docs so future upstream refreshes do not re-add Big Bang sidecar shutdown logic to the upstream toolbox backup Job.
• Adds an Ambient test values file for package validation and bumps the Big Bang package version to 9.11.1-bb.2.
• Adds .DS_Store to .gitignore.

Relevant logs/screenshots

Validated GitLab on the local cluster using:

NAME                    READY   STATUS
gitlab                  True    Helm install succeeded for release gitlab/gitlab.v1 with chart gitlab@9.11.1-bb.2
ztunnel                 True    Helm install succeeded for release istio-system/ztunnel.v1 with chart ztunnel@1.29.1-bb.2
istio-cni               True    Helm install succeeded for release kube-system/istio-cni.v1 with chart istio-cni@1.29.2-bb.0
istiod                  True    Helm install succeeded for release istio-system/istiod.v1 with chart istiod@1.29.2-bb.0
public-ingressgateway   True    Helm install succeeded for release istio-gateway/public-ingressgateway.v1 with chart gateway@1.29.2-bb.0

gitlab-gitaly-0                               1/1   Running
gitlab-gitlab-shell-fb4794df6-ld9hn           1/1   Running
gitlab-minio-766b988d79-rhpzx                 1/1   Running
gitlab-postgresql-0                           1/1   Running
gitlab-redis-master-0                         2/2   Running
gitlab-registry-66fd44f5c9-p6qz6              1/1   Running
gitlab-sidekiq-all-in-1-v2-5d477d6c77-z9l4r   1/1   Running
gitlab-toolbox-6d6f7b9c9d-9z5kn               1/1   Running
gitlab-webservice-default-64ccfb6669-lr26s    2/2   Running

Linked Issue

Closes #399 (closed)

Upgrade Notices

None.