chore: update GitHub templates for LTS

Author: Splaktar

.github/CONTRIBUTING.md

@@ -10,6 +10,11 @@
  - [Developer Commits Levels](#pr_forks)
  - [Submission Guidelines](#submit)
 
+## AngularJS Material is in LTS mode
+
+We are no longer accepting changes that are not critical bug fixes into this project.
+See [Long Term Support](https://material.angularjs.org/latest/#long-term-support) for more detail.
+
 ## <a name="coc"></a> Code of Conduct
 
 Please help us keep AngularJS Material open and inclusive by reading and following our

.github/ISSUE_TEMPLATE/bug.md

@@ -19,6 +19,12 @@ Please submit Angular Material and CDK questions
 [here](https://groups.google.com/forum/#!forum/angular-material2)
 and issues [here](https://github.com/angular/components/issues).
 -->
+# AngularJS Material is in LTS mode
+
+We are no longer accepting changes that are not critical bug fixes into this project.
+See [Long Term Support](https://material.angularjs.org/latest/#long-term-support) for more detail.
+
+
 ## Bug Report
 
 <!-- Please note that we are not accepting major feature requests, i.e. requests for new components, at this time. -->

.github/ISSUE_TEMPLATE/docs.md

@@ -3,6 +3,10 @@ name: Documentation
 about: Suggest an improvement to our documentation at material.angularjs.org
 labels: "type: docs, needs triage"
 ---
+# AngularJS Material is in LTS mode
+
+We are no longer accepting changes that are not critical bug fixes into this project.
+See [Long Term Support](https://material.angularjs.org/latest/#long-term-support) for more detail.
 
 ## Documentation Feedback
 

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,13 +2,21 @@
 Filling out this template is required! Do not delete it when submitting a Pull Request!
 Without this information, your Pull Request may be auto-closed.
 -->
+# AngularJS Material is in LTS mode
+
+We are no longer accepting changes that are not critical bug fixes into this project.
+See [Long Term Support](https://material.angularjs.org/latest/#long-term-support) for more detail.
+
 ## PR Checklist
+
 Please check your PR fulfills the following requirements:
+- [ ] Does this PR fix a regression since 1.2.0, a security flaw, or a problem caused by a new browser version?
 - [ ] The commit message follows [our guidelines](https://github.com/angular/material/blob/master/.github/CONTRIBUTING.md#-commit-message-format)
 - [ ] Tests for the changes have been added or this is not a bug fix / enhancement
 - [ ] Docs have been added, updated, or were not required
 
 ## PR Type
+
 What kind of change does this PR introduce?
 <!-- Please check the one that applies to this PR using "x". -->
 ```