Is it possible that there is no known ECDSA host key for gitlab.com and because of your request for strict checking, the host key verification failed on stderr?

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Yes, it is possible that there is no known ECDSA host key for gitlab.com and that the host key verification failed due to strict checking. This may be because the server's SSH key has recently changed or the key is not trusted by the client. In such cases, the client may prompt the user to verify the authenticity of the server's SSH key before proceeding.