Make announcement milestone a required field for deprecations

data/deprecations/templates/example.yml

@@ -11,6 +11,7 @@
 # Please delete this line and above before submitting your merge request.
 
 - name: "Feature name" # The name of the feature to be deprecated
+  announcement_milestone: "XX.YY" # The milestone when this feature was first announced as deprecated.
   removal_milestone: "XX.YY" # the milestone when this feature is planned to be removed
   body: | # Do not modify this line, instead modify the lines below.
     <!-- START OF BODY COMMENT
@@ -30,6 +31,5 @@
   documentation_url: # (optional) This is a link to the current documentation page
   image_url: # (optional) This is a link to a thumbnail image depicting the feature
   video_url: # (optional) Use the youtube thumbnail URL with the structure of https://img.youtube.com/vi/UNIQUEID/hqdefault.jpg
-  announcement_milestone: # (optional - may be required in the future) XX.YY format - the milestone when this feature was first announced as deprecated
   announcement_date: # (optional - may be required in the future) YYYY-MM-DD format - the date of the milestone release when this feature was first announced as deprecated
   removal_date: # (optional - may be required in the future) YYYY-MM-DD format - the date of the milestone release when this feature is planned to be removed