docs: add release issue template

.github/ISSUE_TEMPLATE/release.md

+---
+name: Release
+about: "*For maintainers only*"
+title: "release: 0.0.0"
+labels: ""
+assignees: "@cdr/code-server-reviewers"
+---
+
+<!-- Maintainer: fill out the checklist -->
+
+## Checklist
+- [ ] Assign to next release manager
+- [ ] Close previous release milestone
+- [ ] Create next release milestone
+- [ ] Associate issue with next release milestone