release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/runtime/doc/ft_rust.txt b/runtime/doc/ft_rust.txt
index d2fc6dd..d5f4080 100644
--- a/runtime/doc/ft_rust.txt
+++ b/runtime/doc/ft_rust.txt
@@ -1,4 +1,4 @@
-*ft_rust.txt*	For Vim version 8.2.  Last change: 2017 Nov 02
+*ft_rust.txt*	For Vim version 9.0.  Last change: 2017 Nov 02
 
 This is documentation for the Rust filetype plugin.