release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/runtime/doc/os_mint.txt b/runtime/doc/os_mint.txt
index d8b516f..4273c55 100644
--- a/runtime/doc/os_mint.txt
+++ b/runtime/doc/os_mint.txt
@@ -1,4 +1,4 @@
-*os_mint.txt*   For Vim version 8.2.  Last change: 2020 Jul 14
+*os_mint.txt*   For Vim version 9.0.  Last change: 2020 Jul 14
 
 
 		  VIM REFERENCE MANUAL    by Jens M. Felderhoff