Use the CSS badge for the API docs as well

Sdoc 2.4.0 supports a CSS badge that can be set by passing the version
as the HORO_BADGE_VERSION env variable.

The old badge image can be removed, as it no longer needs to be copied
by the docs server: https://github.com/rails/rails-docs-server/pull/28

Gemfile

@@ -44,7 +44,7 @@ group :rubocop do
 end
 
 group :doc do
-  gem "sdoc", ">= 2.3.2"
+  gem "sdoc", ">= 2.4.0"
   gem "redcarpet", "~> 3.2.3", platforms: :ruby
   gem "w3c_validators", "~> 1.3.6"
   gem "kindlerb", "~> 1.2.0"

Gemfile.lock

@@ -456,7 +456,7 @@ GEM
     rubyzip (2.3.2)
     rufus-scheduler (3.6.0)
       fugit (~> 1.1, >= 1.1.6)
-    sdoc (2.3.2)
+    sdoc (2.4.0)
       rdoc (>= 5.0)
     selenium-webdriver (4.1.0)
       childprocess (>= 0.5, < 5.0)
@@ -613,7 +613,7 @@ DEPENDENCIES
   rubocop-packaging
   rubocop-performance
   rubocop-rails
-  sdoc (>= 2.3.2)
+  sdoc (>= 2.4.0)
   selenium-webdriver (>= 4.0.0)
   sequel
   sidekiq

railties/lib/rails/api/task.rb

@@ -166,6 +166,7 @@ module Rails
       def setup_horo_variables # :nodoc:
         ENV["HORO_PROJECT_NAME"]    = "Ruby on Rails"
         ENV["HORO_PROJECT_VERSION"] = rails_version
+        ENV["HORO_BADGE_VERSION"]   = badge_version
       end
 
       def api_main
@@ -192,12 +193,20 @@ module Rails
       def rails_version
         "main@#{`git rev-parse HEAD`[0, 7]}"
       end
+
+      def badge_version
+        "edge"
+      end
     end
 
     class StableTask < RepoTask
       def rails_version
         File.read("RAILS_VERSION").strip
       end
+
+      def badge_version
+        "v#{rails_version}"
+      end
     end
   end
 end