[15.0] Remove deprecated DS_DEFAULT_ANALYZERS variable

Summary

DS_DEFAULT_ANALYZERS was deprecated in %14.0 (replaced with DS_EXCLUDED_ANALYZERS). It should be removed as well.

See deprecation issue: #287691 (closed)

Improvements

• Reduce maintenance burden.
• Simplify surface area of supported variables.

Risks

Variable may still be used by users leading to unintended jobs running.

Involved components

• Dependency Scanning template: https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Jobs/Dependency-Scanning.gitlab-ci.yml

Implementation plan

• remove DS_DEFAULT_ANALYZERS from Dependency Scanning template
• remove variable from Dependency Scanning documentation
• remove variable from analyzers and qa templates:
• https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium/-/blob/master/.gitlab-ci.yml
• https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium-maven/-/blob/master/.gitlab-ci.yml
• https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium-python/-/blob/master/.gitlab-ci.yml
• https://gitlab.com/gitlab-org/security-products/ci-templates/raw/master/includes-dev/qa-dependency_scanning.yml