Replace Towncrier with GitHub releases + labels. (#2966)

* Set up GitHub generated release notes.

* Remove Towncrier from the release script.

* Add changelog failures for Labels with Danger and stop requesting a changelog file.

* Update changelog documentation.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,8 +1,8 @@
 ### Pull Request Checklist
 
-- [ ] I read the [contributing guide](https://github.com/element-hq/element-ios/blob/develop/CONTRIBUTING.md)
-- [ ] Pull request contains a [changelog file](https://github.com/matrix-org/matrix-ios-sdk/blob/develop/CONTRIBUTING.md#changelog) in ./changelog.d
-- [ ] Pull request includes a [sign off](https://github.com/matrix-org/matrix-ios-sdk/blob/develop/CONTRIBUTING.md#sign-off)
+- [ ] I read the [contributing guide](https://github.com/element-hq/element-ios/blob/develop/CONTRIBUTING.md).
+- [ ] Pull request contains a [changelog label](https://github.com/element-hq/element-x-ios/blob/develop/CONTRIBUTING.md#changelog).
+- [ ] Pull request includes a [sign off](https://github.com/matrix-org/matrix-ios-sdk/blob/develop/CONTRIBUTING.md#sign-off).
 
 **UI changes have been tested with:**
 - [ ] iPhone and iPad simulators in portrait and landscape orientations.

.github/release.yml

@@ -0,0 +1,31 @@
+changelog:
+  categories:
+    - title: ✨ Features
+      labels:
+        - pr-feature
+    - title: 🙌 Improvements
+      labels:
+        - pr-change
+    - title: 🐛 Bugfixes
+      labels:
+        - pr-bugfix
+    - title: ⚠️ API Changes
+      labels:
+        - pr-api
+    - title: 🗣 Translations
+      labels:
+        - pr-i18n
+    - title: 🧱 Build
+      labels:
+        - pr-build
+    - title: 📄 Documentation
+      labels:
+        - pr-doc
+    - title: 🚧 In development 🚧
+      labels:
+        - pr-wip
+
+    - title: Others
+      labels:
+        - pr-misc
+        - "*"