Jira Cloud fails to connect to self-hosted Gitlab: "Unknown CA"

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Setup: Jira Cloud with "GitLab for Jira (gitlab.com)" v1.0.16-AC + self-hosted Gitlab v15.7

Trying to complete the first-time configuration at https://.atlassian.net/plugins/servlet/ac/gitlab-jira-connect-gitlab.com/gitlab-configuration fails with "Failed to update GitLab version. Please try again.".

A network capture on our self-hosted Gitlab instance shows reproducibly, that the proxy at gitlab.com closes the connection, because it cannot verify our certificate, which is issued by "Sectigo RSA Domain Validation Secure Server CA": "TLSv1.3 Record Layer: Alert (Level: Fatal, Description: Unknown CA)"