release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/runtime/doc/help.txt b/runtime/doc/help.txt
index 2adf651..3615875 100644
--- a/runtime/doc/help.txt
+++ b/runtime/doc/help.txt
@@ -1,4 +1,4 @@
-*help.txt*	For Vim version 8.2.  Last change: 2022 May 13
+*help.txt*	For Vim version 9.0.  Last change: 2022 May 13
 
 			VIM - main help file
 									 k