Update release notes for Gerrit v3.8 and associated links

Final update the release notes of Gerrit v3.8 in preparation
to the announcement on the mailing list.

Contextually perform also the following updates:
- Set Gerrit v3.8 as the latest version on all links
- Declare Gerrit v3.5 as EOL

Change-Id: I3eaab4dc4e3ccb0356ffd367c2d35244d5e96547
6 files changed