mailman: Bump maximum message size for arch-commits to 1000KB

Some of the mails generated by gitlab are way bigger than 200KB, e.g.
this[1] Thunderbird package bump resulting in a 850KB mail. So bump t0
1000KB for now and see if it is enough.

[1] archlinux/packaging/packages/thunderbird@3da91c6e

roles/mailman/defaults/main.yml

@@ -16,7 +16,7 @@ lists:
     description: Arch Linux packaging commits
     display_name: Arch-commits
     info: This list contains all commits to the package repositories, including diffs for newest changes.
-    max_message_size: 200
+    max_message_size: 1000
   arch-dev:
     advertised: false
     archive_policy: private