Add root-namespace-id pod label to Kubernetes executor

What does this MR do?

Adds the project.runner.gitlab.com/root-namespace-id label to Kubernetes job pods, containing the project's root namespace ID.

The label is documented and covered by the Kubernetes executor pod setup tests.

Why was this MR needed?

This allows Kubernetes users and platform integrations to identify and filter job pods based on their GitLab root namespace ID.

What's the best way to test this MR?

Run the Kubernetes executor tests:

go test ./executors/kubernetes -run TestSetupBuildPod -count=1

Verify that the generated pod contains:

project.runner.gitlab.com/root-namespace-id: <root namespace ID>

What are the relevant issue numbers?

gitlab#429739

executors, executorkubernetes