Update link to Secure Coding Guidelines

Why is this change being made?

GitLab Secure Coding Docs link points to Contributor and Development Docs, which does not contain any Secure Coding Docs, but does have a link to another page (Secure Coding Guidelines) that contains this information.

This MR changes the GitLab Secure Coding Docs link to point directly to Secure Coding Guidelines, since it is more relevant in this case.

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct DRI
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to your manager.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
  • If the changes relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files.

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