Change Block Seat Overages Feature Flag to Beta Type

What does this MR do and why?

Change block_seat_overages feature flag to beta type.

The gitlab_com_derisk type made sense originally, but as plans for the feature changed, it became a beta feature.

We need to update the feature flag type accordingly.

Epic: https://gitlab.com/groups/gitlab-org/-/epics/14256?force_legacy_view=true

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/442718

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