Add new template to deploy new stack in staging environment

.gitlab/issue_templates/deploy-new-stack-in-staging.md

+/title Deploy new <project> stack in staging
+
+/milestone %<milestone>
+
+- [ ] swh-apps: Build image (if required)
+- [ ] swh-charts: Bump values to the new version ^
+- [ ] Deploy
+- [ ] Checks
+
+Dashboard to follow [1]
+
+[1] https://grafana.softwareheritage.org/goto/e-c3EAWIz?orgId=1
+
+Refs. <issue>