[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: https://gitlab.com/gitlab-org/gitlab/-/issues/287691
## 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
- [x] remove `DS_DEFAULT_ANALYZERS` from [Dependency Scanning template](https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/ci/templates/Jobs/Dependency-Scanning.gitlab-ci.yml)
- [x] remove variable from [Dependency Scanning documentation](https://docs.gitlab.com/ee/user/application_security/dependency_scanning/#available-cicd-variables)
- [x] remove variable from analyzers and qa templates:
- [x] https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium/-/blob/master/.gitlab-ci.yml
- [x] https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium-maven/-/blob/master/.gitlab-ci.yml
- [x] https://gitlab.com/gitlab-org/security-products/analyzers/gemnasium-python/-/blob/master/.gitlab-ci.yml
- [x] https://gitlab.com/gitlab-org/security-products/ci-templates/raw/master/includes-dev/qa-dependency_scanning.yml
issue