dashboard: disable darwin-amd64-10_11 builder on Go 1.15+

Go 1.15 will require macOS 10.12 and newer¹, so keep the OS X 10.11
builder enabled only on release branches for 1.14 and older.

¹ https://golang.org/doc/go1.14#darwin

Fixes golang/go#37425

Change-Id: I89ab0eb56729fe16fb3a524c8f7b9bb25751d708
Reviewed-on: https://go-review.googlesource.com/c/build/+/222797
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
diff --git a/dashboard/builders_test.go b/dashboard/builders_test.go
index 39e0339..5685708 100644
--- a/dashboard/builders_test.go
+++ b/dashboard/builders_test.go
@@ -522,7 +522,9 @@
 		{b("darwin-amd64-10_15", "go"), onlyPost},
 		{b("darwin-amd64-10_14", "go"), onlyPost},
 		{b("darwin-amd64-10_12", "go"), onlyPost},
-		{b("darwin-amd64-10_11", "go"), onlyPost},
+		{b("darwin-amd64-10_11", "go"), none},
+		{b("darwin-amd64-10_11@go1.14", "go"), onlyPost}, // Go 1.14 is the last release that will run on macOS 10.11 El Capitan.
+		{b("darwin-amd64-10_11@go1.15", "go"), none},     // Go 1.15 will require macOS 10.12 Sierra or later.
 		{b("darwin-386-10_14", "go"), onlyPost},
 		{b("darwin-386-10_14@go1.12", "go"), none},
 		{b("darwin-386-10_14@go1.13", "go"), onlyPost},