fix(KONFLUX-8821): increase rh-sign-image concurrency to 128

[mmalina]

This should help OCP with their ~5000 signing requests.

Describe your changes

Note that this is a workaround. We're waiting for #808 that should help a lot more.

Relevant Jira

KONFLUX-8821

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[openshift-ci]

Skipping CI for Draft Pull Request.
If you want CI signal for your change, please convert it to an actual PR.
You can still manually trigger a test run with /test all

[openshift-merge-robot]

PR needs rebase.

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