[ruby/stringio] Revert "bump up to 3.0.6" [ci skip]

This reverts commit https://github.com/ruby/stringio/commit/325933500b35.
It is bumped to 3.0.5 in advance but not released yet.

https://github.com/ruby/stringio/commit/af67c36693

ext/stringio/stringio.c

@@ -12,7 +12,7 @@
 
 **********************************************************************/
 
-#define STRINGIO_VERSION "3.0.6"
+#define STRINGIO_VERSION "3.0.5"
 
 #include "ruby.h"
 #include "ruby/io.h"