Refine stale bugs - triage policy and plugin

This change adds a policy so that we have a periodic way of pruning inactive stale bugs.

This change has the following:

• Policy to prune stale bugs
• Plugin to identify stale bugs
• Create stale label
• Documentation on the pruning process - gitlab-com/www-gitlab-com!60082 (merged)

Mentions https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/606