cv: target go1.15.
cv: target go1.15.

R=vadimsh, yiwzhang

Bug: 1156241
Change-Id: I8cf22dcd4d59418587106cd382339d7b077e0e9f
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2640336
Commit-Queue: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go