Add next-gen-sast to Secure-Binaries template

What does this MR do and why?

Adds the next-gen-sast analyzer to the offline mirroring template (Security/Secure-Binaries.gitlab-ci.yml) so air-gapped customers can mirror and run the next-gen SAST engine (image next-gen-sast) once it ships as a SAST analyzer.

Modeled on Fix Advanced SAST C/C++ support for offline env... (!242169 - merged) • Hua Yan • 19.2 (which added gitlab-advanced-sast-cpp/clangsa). Because the external SAST job name (next-gen-sast) is identical to the image name, no SECURE_BINARIES_IMAGE_NAME override is needed. The default .download_images behavior (SECURE_BINARIES_IMAGE_NAME: "$CI_JOB_NAME") already resolves to next-gen-sast.

This MR targets the branch of Add gitlab-advanced-sast-ext job to SAST CI tem... (!246271 - merged) • Hua Yan • 19.3, which wires next-gen-sast into the SAST CI templates.

Changes

• lib/gitlab/ci/templates/Security/Secure-Binaries.gitlab-ci.yml: add next-gen-sast to the default SECURE_BINARIES_ANALYZERS list and a next-gen-sast job extending .download_images (version 0, matching the SAST template's SAST_ANALYZER_IMAGE_TAG), gated by a word-boundary rule.
• ee/spec/lib/gitlab/ci/templates/secure_binaries_ci_yaml_spec.rb: add a for next-gen-sast template-content context and a next-gen-sast offline image download job example.
• doc/user/application_security/sast/_index.md: add next-gen-sast:0 to the offline analyzer image list.

Notes

• The standalone "Offline environment" section on a dedicated next-gen SAST docs page (item 3 in the issue plan) is deferred: no next-gen SAST documentation page exists yet, as documentation was intentionally excluded from Add gitlab-advanced-sast-ext job to SAST CI tem... (!246271 - merged) • Hua Yan • 19.3.

Closes https://gitlab.com/gitlab-org/gitlab/-/work_items/606086+s