Automatic Disabling/Deleting users by OIDC removal

This issue has been migrated from [#11500](https://github.com/matrix-org/synapse/issues/11500).

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**Description:**

At the moment we have an Synapse instance connected to gitlab including a filter using the `attribute_requirements:`. I just had to recognize that deleting a user from the defined group does not disable the matrix user. Would it be possible to check if the gitlab user is still in the group and otherwise to disable/delete the matrix user?

Start by reviewing Synapse's OIDC integration and the GitLab attribute_requirements configuration described in the issue. Determine how group-membership removal is detected and whether the expected result is disabling or deleting the Matrix user. Done means the chosen behavior is clearly specified and implemented with coverage for a user removed from the configured GitLab group.

Written by the indexing model from the issue text.