release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/runtime/doc/pi_spec.txt b/runtime/doc/pi_spec.txt
index 35b2210..27d60b8 100644
--- a/runtime/doc/pi_spec.txt
+++ b/runtime/doc/pi_spec.txt
@@ -1,4 +1,4 @@
-*pi_spec.txt*   For Vim version 8.2.  Last change: 2006 Apr 24
+*pi_spec.txt*   For Vim version 9.0.  Last change: 2006 Apr 24
 
 by Gustavo Niemeyer ~