NrUeMac: Change default value of NrUeMac::NumHarqProcess to 16

Someone asked on ns-3-users why the default value is 20. I looked into the 3GPP documents and couldn't find justification for this; it appears that the maximum should be 16 for UL/DL and for SL mode 1 (SL mode 2 has a maximum of 4).

Any concerns about changing this default to 16?