feat: update rollout steps to go from 50 to 100% in one step

What

Remove the roll-out step of 75% so that we go from 50% to 100%.

Also, unrelated, disabled the CKV_GITLAB_2 checkov check otherwise the job fails with this error:

Passed checks: 0, Failed checks: 1, Skipped checks: 0
Check: CKV_GITLAB_2: "Ensure all Gitlab groups require two factor authentication"
	FAILED for resource: /gitlab_conf/groups.json.*./gitlab_conf/groups.json.CKV_GITLAB_2
	File: /groups.json:1-941

We disable this check because 2FA is already enforced at the gitlab-com/gl-infra level.

Why

Google provided the following response after a lengthy case:

Thank you for providing the detailed reproduction of the issue. The product team has carefully reviewed the information and identified that the issue occurs when migrating traffic away from a revision that is assigned less than 50% of the overall traffic and does not have any active instances. We understand that this situation may impact services with sparse traffic, and we sincerely apologize for any inconvenience caused.

To address this, we are actively developing a new and improved implementation of Cloud Run’s traffic migration logic. This upcoming solution is designed to enhance performance and consistency across all traffic scenarios. Given that the issue arises under specific conditions, our focus is on delivering a comprehensive long-term solution rather than a short-term fix. At this time, we are unable to provide a specific ETA for the new implementation, but please be assured that we are committed to resolving this matter thoroughly.

In the interim, you can work around the issue by either:

- Moving all traffic at once, or
- Migrating the last 50% of traffic in a single step.

These approaches should help mitigate the issue for services experiencing sparse traffic. For services with consistent traffic, the incremental traffic migration is expected to work correctly.

We appreciate your patience and understanding as we work to enhance your experience with Cloud Run. If you have any further questions or require additional assistance, please feel free to reach out.

/cc @daveyleach