I’ve just upgrade to GitLab 14.6 which comes with Mattermost 6.1 - update went through without any error message, but Mattermost service is not available. Looking into the log, I’m seeing these a lot:
{"timestamp":"2021-12-22 15:54:04.626 +01:00","level":"fatal","msg":"Failed to alter column type. It is likely you have invalid JSON values in the column. Please fix the values manually and run the migration again.","caller":"sqlstore/store.go:906","error":"pq: default for column \"timezone\" cannot be cast automatically to type jsonb","tableName":"Users","columnName":"Timezone"}
I believe that the GitLab installation uses PostgreSQL (version 12.7 in my case) and I have no idea what I need to be doing to fix this.
EDIT: found the solution in PostgreSQL error when migrating from 5.39.0 to 6.0.0