Adds S/MIME email signing to the release post

Product Marketing: @johnjeremiah | Tech Writer: @rdickenson

Content checklist

• YML filled out for feature
• Description is informative and focuses on the problem we're solving and the value we're delivering
• Documentation is updated and linked
• Screenshot/video is included (optional for secondary items)

Review

When the above is complete and the content is ready for review, it should be reviewed by Product Marketing and Tech Writing:

• PMM review
• Tech writing

Checklist for reference

• Make sure feature description is positive and cheerful
• Check Feature name
• Check Feature availability (Core, Starter, Premium, Ultimate badges)
• Feature is added to data/features.yml (with accompanying screenshots)
• Check all images size < 300KB (compressed, max width 1000 pixels)
• Check that documentation is updated and easy to understand
• Check Documentation links (all feature blocks contain documentation_link)
• Run the content through an automated spelling and grammar check
• Validate all links are functional

References (optional)

• Issue: This is part of a larger effort to improve the security of notifications originating from GitLab - gitlab-org/gitlab#5816
• Release post: %12.3 - !29171 (merged)