add issue template for updating all parts of OAS

.gitlab/issue_templates/update.md

+To update all applications, check the following files:
+
+* [ ] Docker image in `Dockerfile`
+* [ ] requirements.txt
+* [ ] mitogen in `ansible/plugins`
+
+In `group_vars/all/oas.yml`:
+
+* [ ] k3s
+* [ ] helm
+* [ ] krew
+* [ ] velero
+
+Installed by flux:
+
+* [ ] ./kube-system/metallb.yaml
+* [ ] ./kube-system/local-path-provisioner.yaml
+* [ ] ./oas/grafana.yaml
+* [ ] ./oas/letsencrypt-production.yaml
+* [ ] ./oas/single-sign-on.yaml
+* [ ] ./oas/prometheus.yaml
+* [ ] ./oas/letsencrypt-staging.yaml
+* [ ] ./oas/ingress.yaml
+* [ ] ./oas/loki-stack.yaml
+* [ ] ./velero/velero.yaml
+* [ ] ./oas-custom/flux-custom.yaml
+* [ ] ./cert-manager/cert-manager.yaml
+* [ ] ./oas-apps/nextcloud.yaml (make sure to also update our nextcloud helm chart)
+* [ ] ./oas-apps/wordpress.yaml (make sure that our wordpress chart installs the newest version)
+* [ ] ./oas-apps/rocketchat.yaml