maintainers: update roosemberth's mail and add gpg key (#125667)

The previous email account no longer exists.

Signed-off-by: Roosembert Palacios <roosemberth@posteo.ch>

maintainers/maintainer-list.nix

@@ -8778,10 +8778,14 @@
     }];
   };
   roosemberth = {
-    email = "roosembert.palacios+nixpkgs@gmail.com";
+    email = "roosembert.palacios+nixpkgs@posteo.ch";
     github = "roosemberth";
     githubId = 3621083;
     name = "Roosembert (Roosemberth) Palacios";
+    keys = [{
+      longkeyid = "rsa2048/0xCAAAECE5C2242BB7";
+      fingerprint = "78D9 1871 D059 663B 6117  7532 CAAA ECE5 C224 2BB7";
+    }];
   };
   rople380 = {
     name = "rople380";