Cleanup Gardener resources and code

[Sawthis]

Description

When teams will finish migration from Prow to Github actions whole Gardener our automations and accounts won't be needed.

Reasons

We need to cleanup our infrastructure setup after migration from Prow to Github actions.

Acceptance Criteria

• Delete GCP resources connected with Gardener like Cloud runs, SAs and secrets. Update Terraform configs if needed.
• Delete/Remove Gardener accounts/subscriptions.

Attachments

Connected with: #9764

[github-actions]

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Thank you for your contributions.

[Sawthis]

• https://dashboard.garden.live.k8s.ondemand.com/namespace/garden-kyma-prow/shoots was deleted
• all secrets with gardener in name was deleted