[release] LICENSE, extension/LICENSE: fix LICENSE files

The LICENSE files in the release branch maintain the copies of
the LICENSE files embedded in the released products.
This repo hosts two products
  * github.com/golang/vscode-go: source is in .
  * VS Code Go extension (.vsix): source is in ./extension

As we moved the extension code to ./extension (a separate module),
and started to use github.com/golang/vscode-go module to host only
the go program, we needed to rearrange the LICENSE files in the
release branch, but we didn't do it correctly. Fix it now.

* github.com/golang/vscode-go/LICENSE: the license of the
  github.com/golang/vscode-go Go module. Follows the Go module convention.
  Still keep the MIT license.

* github.com/golang/vscode-go/extension/LICENSE: the license of
  the VSIX file that includes all the dependencies licenses. Follows
  the recommendation from our product counsel.

cp extension/LICENSE LICENSE.correct
cp LICENSE extension/LICENSE
mv LICENSE.correct LICENSE

For #3453

Change-Id: Id6caa237d3c783c713081b142fdbd0d6915c23fd
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/598455
Reviewed-by: Robert Findley <[email protected]>
kokoro-CI: kokoro <[email protected]>
Auto-Submit: Hyang-Ah Hana Kim <[email protected]>
Commit-Queue: Hyang-Ah Hana Kim <[email protected]>

Author: hyangah