Disable support for legacy artifacts and traces

What does this MR do?

This introduces two new feature flags:

• ci_enable_legacy_traces,
• ci_enable_legacy_artifacts

That when enabled (disabled by default) provide a support for non-migrated traces and artifacts. Traces either stored on deprecated path or artifacts stored in ci_builds table.

The idea here, is to disable it today. Document what needs to be executed in order to migrate all files, and remove support for non-migrated stuff with %12.0, thus removing feature flags.

What are the relevant issue numbers?

Close https://gitlab.com/gitlab-org/gitlab-ce/issues/56420

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.