Revert "[cv] tentatively downgrade to 1.14 to resolve GAE build failures."
Revert "[cv] tentatively downgrade to 1.14 to resolve GAE build failures."

This reverts commit b88e8019f543e2df734ccab53a78886d701c520a.

Reason for revert: opportunistic upgrade to 1.15.

Original change's description:
> [cv] tentatively downgrade to 1.14 to resolve GAE build failures.
>
> R=​vadimsh, yiwzhang
>
> Change-Id: I63890baedaae97af10741a3d655135f86e4a5882
> Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2770540
> Commit-Queue: Andrii Shyshkalov <tandrii@google.com>
> Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>
> Auto-Submit: Andrii Shyshkalov <tandrii@google.com>
> Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>

Change-Id: I12271cb43b6fde1f3862476ca7e884a3d37f98f0
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2963956
Commit-Queue: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

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