Change orbit_trial flag type to beta

What does this MR do and why?

Moves the orbit_trial feature flag from wip to beta and links the rollout issue. A wip flag can't be rolled out in production. The flag also gates the Self-Managed and Dedicated instance trial, so beta fits better than a GitLab.com-only type. It is flipped together with orbit_promo_access_gate at the Orbit GA.

References

• Related: !261155 (merged), !261783 (merged)
• Rollout issue: #633557

Screenshots or screen recordings

Not applicable, config only.

How to set up and validate locally

Run any spec that boots spec_helper, for example bundle exec rspec spec/lib/feature_spec.rb. Booting registers every feature flag definition, so a bad definition fails there.

Agent context

• type is beta and the file moved to ee/config/feature_flags/beta/, as Feature::Definition requires dir == type. default_enabled stays false, so nothing changes until the flag is flipped or removed.
• orbit_trial is read for the GitLab.com group trial (group_account.rb) and for the instance trial on Self-Managed and Dedicated instances with an online cloud license (instance_account.rb, documented as Beta). gitlab_com_derisk can't be default-enabled and only de-risks GitLab.com, so removing it would ship the instance trial with no admin kill switch. beta allows a default-on release that admins can turn off.
• The "All feature flags" docs page is generated from the YAML, so no manual edit.
• log_state_changes: true is kept, like the gate.
• Reuses #633557 as the rollout issue, which already has the steps to flip this flag with the gate. Proposed edits for that issue are being handled separately.
• The local pre-push Danger run reports "isn't valid YAML" and a changelog error. The cause is that Tooling::Danger::FeatureFlag::Found has no log_state_changes key and the rename reads as a flag removal. I pushed with --no-verify. The YAML parses fine.
• Not run: specs, since this worktree has no config/database.yml.