PR announcement as env variable

This commit changes PR announcement message from hard coded text to env
variable. Thanks to this, it will be possible to change the message in
the repository settings without creating a new PR.

Signed-off-by: Jan Richter <[email protected]>

.github/workflows/pr_announcement.yml

@@ -36,13 +36,7 @@ jobs:
           gh api graphql -f query='mutation {
                   addComment(input: {
                           subjectId: ${{ env.PR_ID }},
-                          body: """Dear contributor,
-                                  Avocado is currently at the end of sprint #105, therefore we are in feature freeze state.
-                                  Please avoid merging changes that do not fall into these categories:
-                                  * Bug fixes
-                                  * Documentation updates
-
-                                  The feature freeze will be active until the release planned on 05/06/2024."""}) {
+                          body: """${{ env.PR_ANNOUNCEMENT }}"""}) {
 
                                   clientMutationId
                   }