devapp: update burndown chart to track 1.15 milestone

Change-Id: I73919249891ad20109511a3bad3fe8d74479ef92
Reviewed-on: https://go-review.googlesource.com/c/build/+/228110
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

devapp/release.go

@@ -27,11 +27,11 @@ const (
 	prefixDev      = "[dev."
 
 	// The title of the current release milestone in GitHub.
-	curMilestoneTitle = "Go1.14"
+	curMilestoneTitle = "Go1.15"
 )
 
 // The start date of the current release milestone.
-var curMilestoneStart = time.Date(2019, 8, 27, 0, 0, 0, 0, time.UTC)
+var curMilestoneStart = time.Date(2020, 2, 21, 0, 0, 0, 0, time.UTC)
 
 // titleDirs returns a slice of prefix directories contained in a title. For
 // devapp,maintner: my cool new change, it will return ["devapp", "maintner"].