[s390x]: Updated job config for release-1.15 for s390x

[dilipgb]

This PR updates the s390x specific job config for release 1.15 for operator, client, serving, eventing and eventing-reconciler. Also fixes the chainguard image issue for s390x platform (Power PR to follow). Removed CI for serving-contour for s390x.

[dilipgb]

@upodroid @dprotaso can you please approve this PR?

[upodroid]

The failure rate is unacceptably high, please track the testgrid boards on a daily basis and resolve failures quickly,

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[knative-prow]

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

• key client-release-1.15.gen.yaml using file prow/jobs/generated/knative/client-release-1.15.gen.yaml
• key eventing-release-1.15.gen.yaml using file prow/jobs/generated/knative/eventing-release-1.15.gen.yaml
• key operator-release-1.15.gen.yaml using file prow/jobs/generated/knative/operator-release-1.15.gen.yaml
• key serving-main.gen.yaml using file prow/jobs/generated/knative/serving-main.gen.yaml
• key serving-release-1.14.gen.yaml using file prow/jobs/generated/knative/serving-release-1.14.gen.yaml
• key serving-release-1.15.gen.yaml using file prow/jobs/generated/knative/serving-release-1.15.gen.yaml

In response to this:

This PR updates the s390x specific job config for release 1.15 for operator, client, serving, eventing and eventing-reconciler. Also fixes the chainguard image issue for s390x platform (Power PR to follow). Removed CI for serving-contour for s390x.

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