Fix crash on startup when using mutter 50.beta

Michael Catanzarorequested to merge
mcatanzaro/keymap-layout-group into main

This symbol was removed in mutter!4699 (merged). Trying to promisify it causes GNOME Shell to crash during initialization:

JS ERROR: Error: [GObject prototype of GIName:Meta.Backend jsobj@0x170d8ebe0a38] has no method named set_keymap_layout_group_async
_promisify@resource:///org/gnome/gjs/modules/core/overrides/Gio.js:469:15
@resource:///org/gnome/shell/ui/environment.js:37:5

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