release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/runtime/doc/usr_05.txt b/runtime/doc/usr_05.txt
index 51e43df..06fb03b 100644
--- a/runtime/doc/usr_05.txt
+++ b/runtime/doc/usr_05.txt
@@ -1,4 +1,4 @@
-*usr_05.txt*	For Vim version 8.2.  Last change: 2019 May 23
+*usr_05.txt*	For Vim version 9.0.  Last change: 2019 May 23
 
 		     VIM USER MANUAL - by Bram Moolenaar