chore: update the PR template now that v3 is officially out (vuejs#2702)

Author: phanan

Diff for: .github/pull_request_template.md

@@ -1,3 +1,3 @@
 Note
 ====
-We're currently in the process of migrating Vue's documentation to v3. To ensure smooth progress, only PR's that fix critical bugs and/or misinformation will be accepted. If yours is not one of them, consider [creating an issue](https://github.com/vuejs/vuejs.org/issues/new) instead and we will label it as `post-3.0` for later tackling.
+This repository is for Vue 1.x and 2.x only. Issues and pull requests related to 3.x are managed in the v3 doc repo: https://github.com/vuejs/docs-next.