Revert "Update links to Gerrit v3.9.0 and set v3.6.x EOL"

This reverts commit 514b8a85b8cc86d6605a5e7752118504a6deaa0e.

Reason for revert: Gerrit v3.9.0 is invalid and people should not use it.

Change-Id: Ia11742f27da711dbf0152354f37b9aa93e56d9a9
4 files changed