Create stale.yml

[ivanahuckova]

This PR adds stale workflow as this repository has a lot of old issues, where we are not certain these are still relevant (some of them are 6+ years old). We've set it up with large time windows (double what we have in grafana/grafana).

For issues:

• we mark issue as stale if no activity in the last 2 years
• we close the issue if no further activity in 2 months

For PRs:

• we mark issue as stale if no activity in the last 60 days
• we close the issue if no further activity in 2 weeks

The automation won't happen for issues/PRs with following labels no stalebot,type/epic,type/bug

[gwdawson]

LGTM