Polaris Cleanup: Triage existing issues

After shipping Polaris, we need to clean up issues:

Evaluate existing issues:
- try to reproduce the issue
- if there's a reproduction, try to do it on the most recent LTS of Ember

Guidance per project:
- Ember Data: if the issue is open, it should be open (regularly triaged)
- Ember CLI: if it's older than 2 years, close the issue
- Ember JS: try to reproduce;
- - if it's an old issue and it doesn't make sense, especially post-Polaris, it's okay to close
- - if it seems to still be an issue, then give it a "still an issue" label
- - if it's a feature we no longer support (e.g. pods) then close the issue with "won't fix"

Phrasing to use when closing issues:

"Unable to reproduce this issue; due to its age, closing issue. Please re-open with reproduction if you feel as though this is still an issue for you."

"This feature is no longer recommended or supported in Ember. Closing issue."