chore: add a migration that removes incorrectly set sysuserid (#6032)

## About the changes

Resets created_by_user_id on events incorrectly marked as -1337 when an actual user has been set in created_by column, to clean up after a bug

src/migrations/20240125090553-events-fix-incorrectly-assigned-sysuser-id.js

@@ -0,0 +1,20 @@
+exports.up = function (db, cb) {
+  db.runSql(
+      `
+      UPDATE events SET created_by_user_id = null
+      WHERE
+          created_by_user_id = -1337 AND
+          created_by NOT IN (
+              'unknown',
+              'migration',
+              'init-api-tokens',
+              'unleash_system_user',
+              'systemuser@getunleash.io');
+      `,
+      cb,
+  );
+};
+
+exports.down = function (db, cb) {
+    cb();
+};