release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/runtime/doc/sponsor.txt b/runtime/doc/sponsor.txt
index 7746609..51d6c16 100644
--- a/runtime/doc/sponsor.txt
+++ b/runtime/doc/sponsor.txt
@@ -1,4 +1,4 @@
-*sponsor.txt*   For Vim version 8.2.  Last change: 2022 May 26
+*sponsor.txt*   For Vim version 9.0.  Last change: 2022 May 26
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar