Adding 16.9.1, 16.8.3, 16.7.6 blog post
Add blog post for 16.9.1, 16.8.3, 16.7.6 patch release.
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Complete any sections of the blog post that have TODO in their content.
Security release tracking issue: https://gitlab.com/gitlab-org/gitlab/-/issues/440842
Copied from https://gitlab.com/gitlab-org/security/www-gitlab-com/-/merge_requests/184