Bump version to 4.0.0rc1 (#19473)

lib/mastodon/version.rb

@@ -5,19 +5,19 @@ module Mastodon
     module_function
 
     def major
-      3
+      4
     end
 
     def minor
-      5
+      0
     end
 
     def patch
-      3
+      0
     end
 
     def flags
-      ''
+      'rc1'
     end
 
     def suffix