mattermost notifications integration triggers a warning about incorrect format

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Summary

When sending notifications from gitlab to mattermost using a webhook you can see this error in the mattermost logs for every notification

mattermost     | {"timestamp":"2026-02-18 14:58:07.479 Z","level":"warn","msg":"Invalid post props. In a future version this will result in an error. Please update your integration to be compliant.","caller":"model/post.go:691","path":"/hooks/rkckn9hsopgozq6cpecgtxnskw","request_id":"cqyb8ppa5fyt7mprzoeraxg3cy","ip_addr":"172.18.0.4","user_id":"","method":"POST","post_id":"nfctxhd5wfynpxufd7oqc34ory","error":"1 error occurred:\n\t* message attachtment at index 0 is invalid: invalid style '#345' - must be one of [good, warning, danger] or a hex color\n\n"}

Steps to reproduce

In mattermost:

• Enable incoming webhooks.
• Add an incoming webhook

In gitlab:

Go to project settings >> integrations >> mattermost notifications >> add the webhook url you got in previous step >> test settings

Tested version

Gitlab v18.8.4 self hosted

mattermost-team-edition:release-11.4