release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/README.txt b/README.txt
index 54f207c..2206088 100644
--- a/README.txt
+++ b/README.txt
@@ -1,4 +1,4 @@
-README.txt for version 8.2 of Vim: Vi IMproved.
+README.txt for version 9.0 of Vim: Vi IMproved.
 
 
 WHAT IS VIM?