Enable Kubernetes command execution with attach by default

Once !1775 (merged) is merged we will have the FF_USE_LEGACY_KUBERNETES_EXECUTION_STRATEGY flag enabled by default, which means GitLab Runner will still use exec to execute remote commands inside Kubernetes containers. The exec strategy is known to cause issues such as #4119 (closed) and #3247 (closed).

In this issue we should decide when the feature flag will be flipped to use attach by default instead of exec.

Related issues to kube attach:

#25346 (closed) #24928 (closed) #25377 (closed)