release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/runtime/doc/pi_gzip.txt b/runtime/doc/pi_gzip.txt
index 19e7387..e5d5ffb 100644
--- a/runtime/doc/pi_gzip.txt
+++ b/runtime/doc/pi_gzip.txt
@@ -1,4 +1,4 @@
-*pi_gzip.txt*   For Vim version 8.2.  Last change: 2019 May 05
+*pi_gzip.txt*   For Vim version 9.0.  Last change: 2019 May 05
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar