Release v2.40.0

What's Changed

• [v2.40.x] Update Google.Protobuf to 3.18.0 by @JamesNK in #1433
• [v2.40.x] Remove load balancing for final release by @JamesNK in #1434
• [v2.40.x] Update version to v2.40.0 by @jtattermusch in #1441

Full Changelog: v2.40.0-pre1...v2.40.0