The RRC uses gNB_ue_ngap_id as its primary ID; the following commits
will introduce the usage of the (F1) gNB CU UE ID. This commits renames
the gNB_ue_ngap_id to rrc_ue_id to better reflect its usage as the RRC's
primary UE ID.

It will be used as the NGAP and F1 (CU) UE IDs.