accept both alive and valid in liveness probe

What does this MR do?

Instead of just accepting alive, both is alive and is valid become acceptable outputs for gitlab-runner verify in the liveness probe.

Why was this MR needed?

When a token is used that was created by the new runner registration workflow, identified by the glrt- prefix, the successful output of gitlab-runner verify becomes Verifying runner... is valid compared to the output when using an old token (Verifying runner... is alive). Due to this difference, liveness probes using new tokens currently fail.

What's the best way to test this MR?

Run two runners, one using the deprecated "runner registration token" and one using the new "runner authentication token". Without this change, the latter has a failing liveness probe, with the change, both should remain healthy.

What are the relevant issue numbers?

Fixes #507 (closed)