chore: temporarily switch back to the old npm disttag (aws#12649)

The publishing of version 2.0.0-alpha.1 is failing because the build is
looking for the npm dist tag 'next' that doesn't yet exist.

This is temporarily switching the tag to use 'preview-v2'. Once the new
version is published, this change will be reverted.


----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

release.json

@@ -1,5 +1,5 @@
 {
   "majorVersion": 2,
   "releaseType": "alpha",
-  "distTag": "next"
+  "distTag": "preview-v2"
 }