extension/package.json: use "explicit" for source.organizeImports

The boolean value is obsolete, and the old 'true' corresponds to
'explicit'.

Reference: microsoft/vscode#194345

Change-Id: I3946682ec840a5d47d89b4b24c6e2fd1341b3ccf
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/613975
kokoro-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hongxiang Jiang <hxjiang@golang.org>
Commit-Queue: Hyang-Ah Hana Kim <hyangah@gmail.com>

extension/package.json

@@ -197,7 +197,7 @@
         "editor.insertSpaces": false,
         "editor.formatOnSave": true,
         "editor.codeActionsOnSave": {
-          "source.organizeImports": true
+          "source.organizeImports": "explicit"
         }
       }
     },