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addition, most of the information was severely outdated, especially with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`
+- [ ] Create and push a signed tag for the version
+- [ ] Build and release DEB and RPM packages
+- [ ] Update bundled Windows dependencies
+- [ ] Build and release Windows packages
+- [ ] Create release on GitHub
+- [ ] Update public docs
+- [ ] Announce release