Fix GitLab merge request state enum

[kempniu]

Reference: https://docs.gitlab.com/ce/api/merge_requests.html#list-merge-requests

[danger-public]

	Fails
🚫	danger-python failed.

Log

Generated by 🚫 dangerJS against dd2df46

[kempniu]

AFAICT, the Danger job for this PR failed (Error: no such option: -p) because Danger Python does not seem to work properly with Danger JS 10.x. (I need to use npm install -g danger@9 in our own CI environment to make Danger Python work.)