Bump version to 1.9.0

HISTORY.md

+## 1.9.0 (2023-01-11)
+
+* [Add support for docutils 0.19, remove support for 0.14/0.15](https://gitlab.com/gitlab-org/gitlab-markup/-/merge_requests/42)
+
+
 ## 1.8.1 (2022-09-16)
 
 * [Fix compatibility with Busybox timeout](https://gitlab.com/gitlab-org/gitlab-markup/-/merge_requests/37)
@@ -21,7 +26,7 @@
 
 ## 1.6.4 (2018-06-08)
 
-* Fixed extra blank line at start of rendered reStructuredText code block 
+* Fixed extra blank line at start of rendered reStructuredText code block
 
 ## 1.6.3 (2017-10-30)
 

lib/github-markup.rb

 module GitHub
   module Markup
-    VERSION = '1.8.1'
+    VERSION = '1.9.0'
     Version = VERSION
   end
 end