Doc: Update repository signing key expriation date

What does this MR do?

Doc: Update repository signing key expriation date

Update the repository signing key expriration date to match as of 2022-03-02, following the actions upon #6701 (closed)

New date: 2024-03-01

Paragraph added to call out how to update key materia

Related issues

#6701 (closed)

gitlab-com/runbooks#38 (moved)

Checklist

See Definition of done.

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Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
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Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened