cv: bump concurrent requests to 50.
cv: bump concurrent requests to 50.

R=qyearsley, yiwzhang

Bug: 1146015
Change-Id: I545841236c2649397384a9bebcddc519c44ed3e3
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2544568
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
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