I would recomend you use a Heavy Forwarder as your HEC endpoint, then send your data on to the Splunk Cloud via normal forwarder method.
A ticket would need to be raised with the Splunk Cloud team, to get the Certificate fixed.
If you do this via a heavy forwarder, look through this section of the manual "AboutsecuringyourSplunkconfigurationwithSSL"
If you would like a good presentation to talk you through setting up, this is a simple guide around the SSL certificate. Best Practices Configuration for Splunk SSL
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