Drop credential.secretRef from Terminal

[petersutter]

What this PR does / why we need it:
This PR removes the credential.secretRef property from both the terminal host and target properties. This feature was originally used for:

• Referencing the Seed's kubeconfig. However, the spec.secretRef field has been removed from the Seed API in g/g without replacement.
• Referencing the Shoot's static token kubeconfig, which was stored as a <shootname>.kubeconfig Secret (for Kubernetes versions below 1.27). With the availability of the shoot/adminkubeconfig API to request admin kubeconfigs for Shoot clusters, referencing the static token kubeconfig Secret is no longer necessary.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:

Release note:

The `credential.secretRef` property has been removed from the terminal `host` and `target` properties:
- For `Seed` resources, `spec.secretRef` was removed from the API without replacement, eliminating the need for `credential.secretRef`.
- For `Shoot` resources, `credential.shootRef` now replaces the previously used `credential.secretRef` for static token kubeconfigs.

[grolu]

/lgtm

[holgerkoser]

/lgtm