chore(github): update PR template to highlight LTS mode

Author: petebacondarwin

Diff for: .github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,11 @@
+# AngularJS is in LTS mode
+We are no longer accepting changes that are not critical bug fixes into this project.
+See https://blog.angular.io/stable-angularjs-and-long-term-support-7e077635ee9c for more detail.
+
 <!-- General PR submission guidelines https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#submit-pr -->
-**What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)**
+**Does this PR fix a regression since 1.7.0, a security flaw, or a problem caused by a new browser version?**
 
+<!-- If the answer is no, then we will not merge this PR -->
 
 
 **What is the current behavior? (You can also link to an open issue here)**