welcome: show Go welcome page only when there is a change user attention is necessary

[hyangah]

Currently the extension shows the welcome page if the extension version the welcome page was shown for is different from the current extension version and the current extension version is one of the versions we explicitly mark to show the welcome page for.

This doesn't handle the case well when the notice applies to all users that install vN+ and we still want to show the notice only once. @findleyr proposed to use a hash of the notice to detect the change and store the hash in the Memento.

For v0.22.1 release, I plan to use a variation - have a hard coded const (v0.22.0) instead of the hash, so users who were prompted when they installed v0.22.0 are not prompted again when they install v0.22.1.

[gopherbot]

Change https://golang.org/cl/289191 mentions this issue: src/goMain: use a hard-coded version string for welcome page state

[gopherbot]

Change https://golang.org/cl/289235 mentions this issue: [release] src/goMain: use a hard-coded version string for welcome page state

[hyangah]

Another request is to not prompt the welcome page in codespace or other dev containers.