Highlight known-issues on Gerrit v3.8

Gerrit v3.8 has been shipped with two known critical issues:
- Change 360646, caused a radical change in the way Gerrit threads
  are handling the updates of the ref-database, leading to potential
  corruption in multi-site and concurrent GCs
- Issue 40015537, causing Gerrit to render a complete blank page on
  certain browsers that have been working fine until v3.7.x

Introduce the "Known issues" section, visually close to the
"Breaking changes" to warn the Gerrit administrator about the potential
breakages that he may encounter when migrating to Gerrit v3.8.x.

The Gerrit administrator can therefore make the necessary adjustments to
the environment and client browsers for preventing the known issues to
impact the users after the upgrade.

Bug: Issue 309098227
Bug: Issue 40015537
Change-Id: I83d7dc34563e381a8b6b7bb82ec820063cc0e16b
1 file changed