dashboard: switch to OpenBSD 6.0 completely, remove OpenBSD 5.8

Updates golang/go#16951

Change-Id: I7b92363671e0ba4c6ab80cef60e7c8600687f440
Reviewed-on: https://go-review.googlesource.com/34384
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
2 files changed