Use `detailed_merge_status` attribute to determine merge status

Ben Brownrequested to merge
benbrown/deprecated-status into main

Unlike the merge_status attribute, this accounts for all potential statuses.

The merge_status attribute has been deprecated, and will be removed in v5 of the API.

https://docs.gitlab.com/ee/api/rest/deprecations.html#merge_status-api-field

Depends on !447 (merged).

Freedesktop SDK's marge-bot deployment has been making use of these changes since 23/08/24.

Edited by Ben Brown

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