feat: add feature-flagged version of Next runtime (#966)

Author: ascorbic

site/plugins.json

@@ -516,6 +516,10 @@
     "repo": "https://github.com/netlify/netlify-plugin-nextjs",
     "version": "4.27.3",
     "compatibility": [
+      {
+        "version": "4.28.1",
+        "featureFlag": "build_plugins_use_prerelease"
+      },
       {
         "version": "4.27.3",
         "migrationGuide": "https://ntl.fyi/next-plugin-migration"