templates: do not use maintainer address in `From` field

Reference: https://lists.debian.org/debian-services-admin/2020/01/msg00000.html

templates/process-unchecked.announce

-From: __MAINTAINER_FROM__
+From: __DAK_ADDRESS__
+Reply-To: __MAINTAINER_FROM__
 To: __ANNOUNCE_LIST_ADDRESS__
 __BCC__
 X-Debian: DAK

templates/process-unchecked.bug-close

-From: __MAINTAINER_FROM__
+From: __DAK_ADDRESS__
+Reply-To: __MAINTAINER_FROM__
 To: __BUG_NUMBER__-close@__BUG_SERVER__
 __BCC__
 X-Debian: DAK