Adjust TBT for project file rendered page

TBT performance of Project File Rendered page was improved under 2000ms in gitlab-org/gitlab#331448 (comment 1543461592)

The MR is to add new performance issue and adjust threshold

changed milestone to %16.4

added Quality maintenancepipelines typemaintenance labels

assigned to @niskhakova

@grantyoung , could you please review?

LGTM!

requested review from @grantyoung

approved this merge request

mentioned in commit a442cf6b

merged