Input contract does no longer contain a valid Seed kubeconfig

[ialidzhikov]

What happened:
After https://github.com/gardener/gardener/pull/8833/files#diff-0c5fc2d4d1cea0541004c8163ef28b926d4ba2702fdc4c143115ecfa992127adL437-L446, the Shoot creation test no longer downloads the Seed kubeconfig. Hence, currently the input contract described in https://github.com/gardener/test-infra/blob/46551fe2356244b8f4bcb3fea2e5f1a2ceff49ac/docs/testmachinery/GetStarted.md#input-contract is not correct for the seed.config file.

See also gardener/gardener-extension-shoot-dns-service#379.

What you expected to happen:
Adapt the docs to remove the details about Seed kubeconfig and adapt stale any handling about the Seed kubeconfig?

How to reproduce it (as minimally and precisely as possible):
N/A

Anything else we need to know:

Environment:

[ialidzhikov]

cc @acumino