doc: Revert the bug reporting address to bugs.gnupg.org

dev.gnupg org is the development platform but the canonical bug
address is and has always been bugs.gnupg.org. We should keep on
using this address for the case that we switch the tracker again or
split it off the development system.

That is also the reason why we should keep on communicating a plain
bug number without the 'T' prefix.

• Signed-off-by: Werner Koch <wk@gnupg.org>