release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/runtime/doc/os_dos.txt b/runtime/doc/os_dos.txt
index b315be8..e5e405c 100644
--- a/runtime/doc/os_dos.txt
+++ b/runtime/doc/os_dos.txt
@@ -1,4 +1,4 @@
-*os_dos.txt*    For Vim version 8.2.  Last change: 2006 Mar 30
+*os_dos.txt*    For Vim version 9.0.  Last change: 2006 Mar 30
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar