Update release issue template
Created by: lguychard
As discussed in the 3.8 retrospective:
- Mention that release testing grid creation and assignments should be done at the beginning of the iteration (vs. 5 days before the release).
- Mention that high-level details should be added to issues found during release testing to help Product determine whether the issue is a release blocker.
- Mention that, in the days before the release, the release captain should aggressively revert features that may cause delays.