fix: maintainer_can_modify for forked PRs

Closes #2476

lib/platform/github/index.js

@@ -798,6 +798,7 @@ async function createPr(
   // istanbul ignore if
   if (config.forkToken) {
     options.token = config.forkToken;
+    options.body.maintainer_can_modify = true;
   }
   logger.debug({ title, head, base }, 'Creating PR');
   const pr = (await get.post(