Add 17.6 What's new content

What does this MR do and why?

This MR adds 7.6 release What's new content (see process docs).

It should be merged AFTER the release post is live on the release date, 11-21-2024.

UPDATE release post is live: https://about.gitlab.com/releases/2024/11/21/gitlab-17-6-released/

This MR can be merged.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• https://handbook.gitlab.com/handbook/marketing/blog/release-posts/#creating-an-mr-for-whats-new-entries

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