Patch greeter.dconf-defaults instead of shipping a full fork

Commit 6a2931f introduces a full fork of greeter.gsettings, without any explanation of why it needs to be done this way. This will be hard to maintain, and may cause hard to debug issues. Please convert this back to a chroot_local-patches.

Parent Task: #7561 (closed)

Related issues

Related to #7599 (closed)

Original created by @intrigeri on 7637 (Redmine)

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