Upgrade release process to v2 only

* [x] stop publishing v2 docs #1042

These probably should be done after actually doing the v2-only release
* [ ] remove v1 artifacts from Download page (yaml and template)
* [x] remove all older v1 versions except for the latest v1 (from `data/`, `content/`, and `hugo.yaml`)
* [x] remove content/v1/_dev and make sure it doesn't break the build

The remaining unchecked work concerns the Download page's v1 artifacts in its YAML and template. Locate those files, remove the v1 entries, and verify that the documentation build still succeeds; the issue's completed cleanup references data/, content/, hugo.yaml, and content/v1/_dev.

Written by the indexing model from the issue text.