qt: deprecate support for >= macOS Sierra

Formula/qt.rb

@@ -1,16 +1,13 @@
 class Qt < Formula
   desc "Cross-platform application and UI framework"
   homepage "https://www.qt.io/"
+  url "https://download.qt.io/official_releases/qt/4.8/4.8.7/qt-everywhere-opensource-src-4.8.7.tar.gz"
+  mirror "https://www.mirrorservice.org/sites/download.qt-project.org/official_releases/qt/4.8/4.8.7/qt-everywhere-opensource-src-4.8.7.tar.gz"
+  sha256 "e2882295097e47fe089f8ac741a95fef47e0a73a3f3cdf21b56990638f626ea0"
   revision 2
 
   head "https://code.qt.io/qt/qt.git", :branch => "4.8"
 
-  stable do
-    url "https://download.qt.io/official_releases/qt/4.8/4.8.7/qt-everywhere-opensource-src-4.8.7.tar.gz"
-    mirror "https://www.mirrorservice.org/sites/download.qt-project.org/official_releases/qt/4.8/4.8.7/qt-everywhere-opensource-src-4.8.7.tar.gz"
-    sha256 "e2882295097e47fe089f8ac741a95fef47e0a73a3f3cdf21b56990638f626ea0"
-  end
-
   bottle do
     sha256 "f6dc9df6f78e1d8c12ebf961c8a9196885a1ee732eed098b2cbe8320f2d9a7a8" => :el_capitan
     sha256 "323bcba88bd3600a4a5dc26d43602e57a71609f1d9a620b9d42b63426569e191" => :yosemite
@@ -33,6 +30,11 @@ class Qt < Formula
   depends_on "mysql" => :optional
   depends_on "postgresql" => :optional
 
+  # Qt4 is dead upstream. We backported a build fix for 10.11 but do not
+  # intend to keep rescuing it forever, including for macOS 10.12. Homebrew will
+  # be migrating to Qt5 as widely as possible, which remains supported upstream.
+  depends_on MaximumMacOSRequirement => :el_capitan
+
   deprecated_option "qtdbus" => "with-dbus"
   deprecated_option "with-d-bus" => "with-dbus"