Enable bso_minimal_access_fallback feature flag by default

What does this MR do and why?

Promotes the bso_minimal_access_fallback feature flag from wip to beta and enables it by default.

This feature flag controls the behavior of falling back to Minimal Access when provisioning users through SAML, SCIM, or LDAP when Block Seat Overages (BSO) is enabled and no seats are available.

With !216727 (merged) now merged, minimal access users are non-billable across all tiers (including self-managed Premium), making this fallback behavior safe to enable by default.

References

• https://gitlab.com/gitlab-org/gitlab/-/issues/513219+
• Add foundation module for BSO seat check (!201940 - merged)
• Make minimal access users non-billable (!216727 - merged)

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