release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/runtime/doc/usr_45.txt b/runtime/doc/usr_45.txt
index e93f747..7e919a9 100644
--- a/runtime/doc/usr_45.txt
+++ b/runtime/doc/usr_45.txt
@@ -1,4 +1,4 @@
-*usr_45.txt*	For Vim version 8.2.  Last change: 2022 May 13
+*usr_45.txt*	For Vim version 9.0.  Last change: 2022 May 13
 
 		     VIM USER MANUAL - by Bram Moolenaar