CHANGELOG.md: add release heading for v0.48.0

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

Change-Id: I8848b7353295d9463286e9d249a23ac47dd7c44b
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/684855
kokoro-CI: kokoro <[email protected]>
Reviewed-by: Hongxiang Jiang <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Gopher Robot <[email protected]>
Reviewed-by: Madeline Kalil <[email protected]>

Author: gopherbot

CHANGELOG.md

@@ -5,6 +5,10 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/).
 
 ## Unreleased
 
+## v0.48.0
+
+Date: 2025-06-30
+
 ### Changes
 
 * Introduced quick pick separator in command `Go: Choose Go Environment` showing