gopls: update to the latest version when we turn it on by default

[stamblerre]

Users who have opted out of gopls may have outdated versions on their machines. We should not delay before prompting them to update--we should update right away, before we turn it on.

[hyangah]

This is critical for the first user experience.

[gopherbot]

Change https://golang.org/cl/274250 mentions this issue: src/goLanguageServer.ts: update gopls when gopls is turned on by default

[stamblerre]

Can we close this now that https://golang.org/cl/274250/ is merged, or is there other work to do here?

[hyangah]

Thanks @suzmue !