Update release notes for new gh ado2gh extension

Author: dylan-smith

RELEASENOTES.md

@@ -1 +1,4 @@
-- Hide ADO specific args from `gh gei` in favor of `gh ado2gh`.
+- We are now shipping all functionality for Azure DevOps -> GitHub migrations as a new extension to the GitHub CLI. Run `gh extension install github/gh-ado2gh` to install. All commands are the same as the previous `ado2gh` CLI (e.g. `gh ado2gh generate-script`). If you were previously using `gh gei` to handle ADO migrations (instead of the separate `ado2gh` stand-alone CLI), that functionality has now been moved to the `gh ado2gh` extension. `gh gei` is solely used for GitHub -> GitHub migration scenarios.
+  - We will continue shipping `ado2gh` stand-alone CLI in this release and probably the next couple, after that the only way to acquire new versions will be via the `gh` extension.
+  - Technically in this release the ADO capabilities in `gh gei` have only been hidden from the built-in help, but will still work and give the user a warning that they should be using `gh ado2gh` instead. In a future release we will remove them entirely.
+  - Our docs have been updated to reflect these changes and can be accessed here: https://docs.github.com/en/early-access/enterprise-importer