Update default deprecation horizon

Because the master branch is already 6.0.

activesupport/lib/active_support/deprecation.rb

@@ -35,7 +35,7 @@ class Deprecation
     # and the second is a library name.
     #
     #   ActiveSupport::Deprecation.new('2.0', 'MyLibrary')
-    def initialize(deprecation_horizon = "6.0", gem_name = "Rails")
+    def initialize(deprecation_horizon = "6.1", gem_name = "Rails")
       self.gem_name = gem_name
       self.deprecation_horizon = deprecation_horizon
       # By default, warnings are not silenced and debugging is off.