Restore external gitlab-flow redirect removed too soon

What does this MR do?

This gitlab_flow file was originally planned to be an internal redirect, but at the end of the project, we determined it was better moved to a blog post. External redirects are kept for a year, not 3 months, so we need to restore the redirect until the original year-from-removal.

We missed the need to change the redirect from 3 months to a year. This MR restores the redirect with the 2024 removal date it should've had from the start.

Related issues

• Related to https://gitlab.com/gitlab-com/customer-success/professional-services-group/education-services/-/issues/730+ where the too-soon redirect removal was found
• Related to Redact GitLab Flow page, create a redirect (!127638 - merged) where the redirect was merged with a 3-month limit
• Related to Gitlab Flow Docs - Adding Enablement Content fo... (#378297 - closed) which tracks the full arc of problems with this page over the years
• Related to Blog Post - The GitLab Flow with GitLab Duo: a ... (gitlab-com/www-gitlab-com!127409 - merged) where the blog post was published

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