Replace `RELEASE.md' with issue template

Apart from the Windows-related notes, all instructions are common to our
releases and therefore do not need to be explicitly listed here. In
addition, most of the information was severely outdated, especially with
respect to how our packaging works.

.github/ISSUE_TEMPLATE/release.md

@@ -0,0 +1,21 @@
+---
+name: '[INTERNAL] Release'
+about: Release a version
+title: 'Release Version v$version'
+labels: ''
+assignees: ''
+
+---
+
+# Release Workflow
+
+- [ ] Check that the `.mailmap` and `AUTHORS` files are up to date
+- [ ] Update `ICINGA2_VERSION`
+- [ ] Update `CHANGELOG.md`
+- [ ] Update bundled Windows dependencies
+- [ ] Create and push a signed tag for the version
+- [ ] Build and release DEB and RPM packages
+- [ ] Build and release Windows packages
+- [ ] Create release on GitHub
+- [ ] Update public docs
+- [ ] Announce release