Store lastInstalledExtensionVersion in the global storage so we onl…

…y show `what's new` once (#971)

## Changes
* We were storing `lastInstalledExtensionVersion` in workspace state
storage, which is causing the extension to show the `what's new` popup
for every workspace once. We should store it in the global storage
instead.

**Note**: Users will see a 1 time popup after this update before the
versions are updated in the global location correctly.

## Tests
* manually

packages/databricks-vscode/src/vscode-objs/StateStorage.ts

@@ -70,7 +70,7 @@ const Keys = {
     }),
 
     "databricks.lastInstalledExtensionVersion": withType<string>()({
-        location: "workspace",
+        location: "global",
         defaultValue: "0.0.0",
     }),
 };