release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/runtime/doc/pi_zip.txt b/runtime/doc/pi_zip.txt
index 787ca4d..d5a71b7 100644
--- a/runtime/doc/pi_zip.txt
+++ b/runtime/doc/pi_zip.txt
@@ -1,4 +1,4 @@
-*pi_zip.txt*	For Vim version 8.2.  Last change: 2021 Nov 08
+*pi_zip.txt*	For Vim version 9.0.  Last change: 2021 Nov 08
 
 				+====================+
 				| Zip File Interface |