Update secure_coding_guidelines.md

What does this MR do and why?

Background: a prospective customer who requires the most up-to-date encryption mechanisms, was confused about the language regarding TLS versioning in the documentation (ie whether GitLab allowed the use of older TLS versions).

This update more directly states that only TLS 1.2 and above can be used inside GitLab.

Describe in detail what your merge request does and why.

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MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.