Move Security Attributes release post to 18.9 branch

Moves the Security Attributes (GA) release post from unreleased to the 18.9 release branch and updates the image URL path accordingly.\n\n- Moved data/release_posts/unreleased/security-risk-management-security-attributes-ga.yml to data/release_posts/18_9/security-risk-management-security-attributes-ga.yml\n- Updated image_url from /images/unreleased/security-attributes.png to /images/18_9/security-attributes.png"