Fix most references to Secure

Why is this change being made?

The Secure Stage was renamed to Application Security Testing last year.

Some of the sections are broken because of incorrect references. I noticed that https://handbook.gitlab.com/handbook/engineering/development/sec/secure/#sub-department-development-people-leaders was missing @amarpatel, and in the effort to fix it, I ended-up updating more than I set-out to do.

Companion MR: gitlab-com/www-gitlab-com!138545 (merged)

Author and Reviewer Checklist

Please verify the check list and ensure to tick them off before the MR is merged.

• Provided a concise title for this Merge Request (MR)
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Commits

• Fix most references to Secure

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