release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/runtime/doc/os_haiku.txt b/runtime/doc/os_haiku.txt
index a22b99b..be08b4a 100644
--- a/runtime/doc/os_haiku.txt
+++ b/runtime/doc/os_haiku.txt
@@ -1,4 +1,4 @@
-*os_haiku.txt*	For Vim version 8.2.  Last change: 2020 May 13
+*os_haiku.txt*	For Vim version 9.0.  Last change: 2020 May 13
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar