Clean up ClusterInfo struct and fix nil panic

[YiqinZhang]

Remove unused Name, Region, CloudProvider fields from slack.ClusterInfo,
Align sendFailureNotification ClusterInfo construction with sendDeferredNotifications for consistency;
Add nil guards in sendDeferredNotifications to prevent panic when provider or cluster is unavailable.

SDCICD-1770

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@YiqinZhang: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/code-quality-checks	3e3a1b3	link	true	/test code-quality-checks

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