feat: Bump major security-code-scan sast version for 15.0

What does this MR do and why?

The major version of SAST's security-code-scan analyzer is scheduled to bump to 3 in 15.0. To avoid coordination of the deployment, we add a conditional on CI_SERVER_VERSION_MAJOR to set the major version to 3 automatically

Relates to #350935 (closed)

How to set up and validate locally

Sanity check to ensure v2 is still retrieved: https://gitlab.com/theoretick/csharp-dotnetcore-multiproject-bump-sast-scs-major-version-to-3/-/jobs/2047339154

Hard to sanity-check the alternative until major version is bumped but see 925cee67 where I added a spec mocking the major version.

MR acceptance checklist

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