Promote credit_caps_bulk_update FF to beta and enable by default

What does this MR do and why?

Promotes the credit_caps_bulk_update feature flag from wip to beta and sets default_enabled: true, so bulk CSV import of per-user credit cap overrides is available on self-managed instances without enabling the flag manually.

⚠️ Before merging: disable the flag on SaaS as described in the issue (chatops in #production):

chatops gitlab run feature set credit_caps_bulk_update false

References

• Closes https://gitlab.com/gitlab-org/gitlab/-/work_items/632189
• Feature: https://gitlab.com/gitlab-org/gitlab/-/work_items/630981
• Rollout: https://gitlab.com/gitlab-org/gitlab/-/work_items/631018

Screenshots or screen recordings

N/A, this only changes the feature flag definition.

How to set up and validate locally

N/A

MR acceptance checklist

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