extension: update gopls v0.20.0-pre.1 settings

This is an automated CL which updates the gopls version and settings.

For golang/go#74655

Change-Id: I4e99b890e321533d29a781b7554074452cb22cd0
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/688555
kokoro-CI: kokoro <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>
Auto-Submit: Gopher Robot <[email protected]>
Reviewed-by: Hongxiang Jiang <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: gopherbot