package.json: upgrade to the latest vscode-languageclient pre-release

This includes the fix for
microsoft/language-server-protocol#1107. We
need this merged in order to resolve golang/go#42148.

Change-Id: I675b20f24019f590f8c3dbb4257c8414d282de98
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/266497
Trust: Rebecca Stambler <rstambler@golang.org>
Trust: Suzy Mueller <suzmue@golang.org>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Suzy Mueller <suzmue@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

package.json

@@ -56,7 +56,7 @@
     "vscode-debugadapter": "^1.40.0",
     "vscode-debugadapter-testsupport": "^1.42.0",
     "vscode-debugprotocol": "^1.40.0",
-    "vscode-languageclient": "^7.0.0-next.9",
+    "vscode-languageclient": "^7.0.0-next.12",
     "vscode-languageserver-protocol": "^3.16.0-next.7",
     "web-request": "^1.0.7"
   },