release version 9.0

Problem:    About time to release Vim 9.0.
Solution:   Update the version number everywhere.
diff --git a/READMEdir/README_os2.txt b/READMEdir/README_os2.txt
index 3a9cdf1..19b6a53 100644
--- a/READMEdir/README_os2.txt
+++ b/READMEdir/README_os2.txt
@@ -1,4 +1,4 @@
-README_os2.txt for version 8.2 of Vim: Vi IMproved.
+README_os2.txt for version 9.0 of Vim: Vi IMproved.
 
 This file used to explain the installation of Vim on OS/2 systems.
 However, support for OS/2 has been removed in patch 7.4.1008.