Add tar as explicit runtime dependency and update source install docs

Summary

Follow-up to #585695 (closed): Add tar as an explicit runtime dependency in omnibus-gitlab and ensure it's documented in source install requirements.

Problem to solve

While tar is typically assumed to be available on most systems, it should be explicitly documented and added as a runtime dependency to ensure it's available for GitLab operations like project imports.

Proposed solution

1. Update source install documentation at https://docs.gitlab.com/install/self_compiled/#software-requirements to include tar as a required system utility.

2. Add tar as runtime dependency in omnibus-gitlab at https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/master/config/projects/gitlab.rb using a runtime_dependency statement, similar to how other system utilities are declared.

Implementation details

• Add tar to the software requirements section of the source installation guide
• Work with the build team to add the explicit runtime dependency declaration in omnibus-gitlab's gitlab.rb configuration
• Ensure the dependency is properly documented so Linux package users understand it's a required system utility

Related issues

• #585695 (closed) - add information that tar needs to be installed on the import server before the import can be started

Notes

As discussed: "That is exactly the reason. But nothing is preventing us from making it an explicit dependency via a runtime_dependency statement in omnibus-gitlab's config/projects/gitlab.rb."