Disable GDM debug logs

For #12092 (closed), we’re going to ship with GDM debug logging enabled in 4.0~beta2, to make it easier to debug any regression introduced by that branch.
We should revert this in 4.0: config/chroot_local-hooks/30-gdm-debug.

Feature Branch: bugfix/17011-disable-gdm-debug-logs

Related issues

Related to #12092 (closed)
Blocks #16209

Original created by @intrigeri on 17011 (Redmine)

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