It’s a warning caused by an semi-experimental deprecation in php 7.3. To eliminate the warning simply follow its suggestion - and the explicit “continue 2” is supported in previous php versions and clarifies the intent anyway which is always A Good Thing.
So editing the relevant lines (1 each) in the files:
/3rdparty/sabre/vobject/lib/Component/VCard.php
/3rdparty/sabre/dav/lib/CalDAV/ICSExportPlugin.php
and replacing the appearance of “continue;” with “continue 2;” should be a long-term fix.