release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/runtime/doc/diff.txt b/runtime/doc/diff.txt
index fad6801..a66df1e 100644
--- a/runtime/doc/diff.txt
+++ b/runtime/doc/diff.txt
@@ -1,4 +1,4 @@
-*diff.txt*      For Vim version 8.2.  Last change: 2022 Jan 22
+*diff.txt*      For Vim version 9.0.  Last change: 2022 Jan 22
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar