Update base elixir version to 1.9.4

What does this MR do?

Update base image elixir version from 1.7.4 to 1.9.4

For regression testing I've created an elixir-1.7-FREEZE branch within our test project.

This MR has been tested against https://gitlab.com/gitlab-org/security-products/tests/elixir-phoenix w/ SAST_DEFAULT_ANALYZERS=sobelow & SAST_ANALYZER_IMAGE_TAG=update-elixir-base-to-1.9.4: Example job https://gitlab.com/gitlab-org/security-products/tests/elixir-phoenix/-/jobs/356565685

What are the relevant issue numbers?

Fixes gitlab-org/gitlab#36518 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10527
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Auto-DevOps template
  • Job definition example
  • CI Templates
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer