Update mattermost SSO config to use oidc instead of gitlab method

[jacobbmay]

Currently we are using the gitlab auth method and using some hacks to make it work with keycloak. This is an issue since users created directly in the console or through an AD sync don't currently get mattermost IDs generated by our plugin.

Enterprise mattermost supports OIDC logins so we should be able to set mattermost up to auth directly with Keycloak without tricking it into thinking it is talking to GitLab so we should be able to avoid needing the mattermost ID field from our plugin.

This will require a mattermost enterprise license. We can start testing with a trial license, but will need a permanent solution after 30 days of getting the trial license.