release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/runtime/doc/syntax.txt b/runtime/doc/syntax.txt
index e75f870..c3d6777 100644
--- a/runtime/doc/syntax.txt
+++ b/runtime/doc/syntax.txt
@@ -1,4 +1,4 @@
-*syntax.txt*	For Vim version 8.2.  Last change: 2022 Jun 10
+*syntax.txt*	For Vim version 9.0.  Last change: 2022 Jun 10
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar