add new deprecated field to release notes yaml

Author: sarahs

data/release-notes/3-0/0-rc1.yml

@@ -1,5 +1,6 @@
 date: '2021-01-12'
 release_candidate: true
+deprecated: true
 intro: Release candidate versions should be tested on non-production environments. For more information about the Release Candidate Program, see the [GitHub Blog](https://github.blog/2020-12-03-improving-the-ghes-release-process-release-candidates/) or "[About upgrades to new releases](/admin/overview/about-upgrades-to-new-releases)".
 sections:
   bugs:

data/release-notes/3-0/0-rc2.yml

@@ -1,5 +1,6 @@
 date: '2021-01-29'
 release_candidate: true
+deprecated: true
 intro: Release candidate versions should be tested on non-production environments. For more information about the Release Candidate Program, see the [GitHub Blog](https://github.blog/2020-12-03-improving-the-ghes-release-process-release-candidates/) or "[About upgrades to new releases](/admin/overview/about-upgrades-to-new-releases)."
 sections:
   bugs:

lib/release-notes-schema.js

@@ -37,6 +37,10 @@ module.exports = {
       type: 'boolean',
       default: false
     },
+    deprecated: {
+      type: 'boolean',
+      default: false
+    },
     sections: {
       required: true,
       type: 'object',