[s390x] add release 1.8 job for operator

[haubenr]

This PR is the follow up to https://github.com/knative/test-infra/pull/3581 and adjusts the test job schedule for operator release 1.8 for platform s390x.

[knative-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: haubenr, upodroid

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[knative-prow[bot]]

@haubenr: Updated the job-config configmap in namespace default at cluster default using the following files:

• key operator-release-1.8.gen.yaml using file prow/jobs/generated/knative/operator-release-1.8.gen.yaml

In response to [this](https://github.com/knative/test-infra/pull/3583): >This PR is the follow up to https://github.com/knative/test-infra/pull/3581 and adjusts the test job schedule for operator release 1.8 for platform s390x. > Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.