release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/runtime/doc/testing.txt b/runtime/doc/testing.txt
index 42246c0..1252ce3 100644
--- a/runtime/doc/testing.txt
+++ b/runtime/doc/testing.txt
@@ -1,4 +1,4 @@
-*testing.txt*	For Vim version 8.2.  Last change: 2022 May 16
+*testing.txt*	For Vim version 9.0.  Last change: 2022 May 16
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar