d/control: Set maintainers to librem5-maintainers

debian/control

 Source: librem5-base
 Section: metapackages
 Priority: optional
-Maintainer: Guido Günther <agx@sigxcpu.org>
+Maintainer: librem5-maintainers <librem-5-dev@lists.community.puri.sm>
+Uploaders:
+ Guido Günther <agx@sigxcpu.org>
 Build-Depends:
  debhelper (>= 10),
  germinate (>= 1.18),