[release] package.json: remove 'preview' attribute

Release branch should host the source for the stable version.
This patch will remain only in release & latest branches.

Change-Id: Ib4fa0ed46fbce2e55e8b1eb8266625bbc5723ff2
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/284094
Trust: Hyang-Ah Hana Kim <[email protected]>
Run-TryBot: Hyang-Ah Hana Kim <[email protected]>
TryBot-Result: kokoro <[email protected]>
Reviewed-by: Suzy Mueller <[email protected]>

Author: hyangah

package.json

@@ -2,7 +2,6 @@
   "name": "go",
   "displayName": "Go",
   "version": "0.21.0",
-  "preview": true,
   "publisher": "golang",
   "description": "Rich Go language support for Visual Studio Code",
   "author": {