cs: fix banner and make startup use it

The string returned by `banner` was missing a "v" and a "." compared
to the traditional result. Avoid duplicating the string in "main.sps".

racket/src/cs/main.sps

@@ -760,7 +760,7 @@
           (apply orig args)))))
 
    (when version?
-     (printf "Welcome to Racket v~a [cs].\n" (version)))
+     (display (banner)))
    (call-in-main-thread
     (lambda ()
       (initialize-place!)

racket/src/cs/rumble/version.ss

@@ -22,4 +22,4 @@
                (loop 0)]))]))))))
 
 (define (version) (extract-version-string))
-(define (banner) (string-append "Welcome to Racket " (version) "\n"))
+(define (banner) (string-append "Welcome to Racket v" (version) " [cs].\n"))