fix: reduce embedded changelogs to last 15

lib/workers/pr/index.js

@@ -140,7 +140,7 @@ async function ensurePr(prConfig) {
         });
       }
       // istanbul ignore next
-      for (let i = 20; i < upgrade.releases.length; i += 1) {
+      for (let i = 15; i < upgrade.releases.length; i += 1) {
         if (upgrade.releases[i].releaseNotes) {
           upgrade.releases[i].releaseNotes.body = null;
         }