Migrate mc18 HI test to mc21, using ttbar and removing hijing from HITtoRDO step.

Mark Hodgkinsonrequested to merge
mhodgkin/athena:mhodgkin_ATR_27107 into 23.0

This is a workaround for ATR-27107 until new HI HITS files are available which can work in 23/master. Also relevant for ATLASRECTS-7555

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