Dismissible alert to announce the navigation toggle removal

Release notes

In 16.6 we are removing the old navigation and ability to go back.

Proposal

Text for the announcement

Logic: If they are logged in and do not have the new navigation enabled

[DRAFT] GitLab has redesigned the left sidebar to address customer feedback. View details in [this blog post]. Here's how to [file issues](https://about.gitlab.com/submit-feedback/#product-feedback) with the GitLab product team.

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