Correct version to 1.2.1: v1.2.0 was already tagged for load balancing

The load-balancing feature (merged in #172/#173) was tagged v1.2.0 on
GitHub, but nodejs/package.json and CHANGELOG.md were never bumped for
it. Backfill a 1.2.0 CHANGELOG entry and bump this release to 1.2.1
instead of 1.1.18, so package.json/CHANGELOG stay ahead of the highest
existing tag rather than behind it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

nodejs/package.json

@@ -1,6 +1,6 @@
 {
   "name": "proxy-api",
-  "version": "1.1.18",
+  "version": "1.2.1",
   "author": [
     {
       "name": "William Mantly",