Key rollover: add a checklist item for the Download page.

Next time I do a release, I'll have to remember to adjust the download
page links to the GPG signature files.

(cherry picked from commit 7524da621b)

CHECKLST.txt

@@ -112,6 +112,11 @@ for it:
    + .htaccess has an individual redirect for each version number. Add
      a new one.
 
+ - For 0.66 only: if it's not already done, switch the remaining
+   signature links on the Download page over to using the new
+   signature style. Then remove this checklist item, since it'll only
+   need doing this once.
+
  - If there are any last-minute wishlist entries (e.g. security
    vulnerabilities fixed in the new release), write entries for them
    in a local checkout of putty-wishlist.