CHANGELOG.md: add release heading for v0.50.0

This is an automated CL which updates the CHANGELOG.md.

Change-Id: Id54e0ca41f180c529929d16406246b2ca5ad1313
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/700995
Reviewed-by: Hongxiang Jiang <[email protected]>
Auto-Submit: Gopher Robot <[email protected]>
Reviewed-by: Madeline Kalil <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: gopherbot

CHANGELOG.md

@@ -5,6 +5,10 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/).
 
 ## Unreleased
 
+## v0.50.0
+
+Date: 2025-09-04
+
 ## v0.49.1 (prerelease)
 
 Date: 2025-08-21