release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/runtime/doc/usr_21.txt b/runtime/doc/usr_21.txt
index 953a3fb..2f6ee9c 100644
--- a/runtime/doc/usr_21.txt
+++ b/runtime/doc/usr_21.txt
@@ -1,4 +1,4 @@
-*usr_21.txt*	For Vim version 8.2.  Last change: 2019 Apr 25
+*usr_21.txt*	For Vim version 9.0.  Last change: 2019 Apr 25
 
 		     VIM USER MANUAL - by Bram Moolenaar