release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/runtime/doc/usr_12.txt b/runtime/doc/usr_12.txt
index 7781f8e..68f6cbe 100644
--- a/runtime/doc/usr_12.txt
+++ b/runtime/doc/usr_12.txt
@@ -1,4 +1,4 @@
-*usr_12.txt*	For Vim version 8.2.  Last change: 2021 Apr 19
+*usr_12.txt*	For Vim version 9.0.  Last change: 2021 Apr 19
 
 		     VIM USER MANUAL - by Bram Moolenaar