fix NTN LEO scenarios

• gNB: add support for NTN parameter ta-CommonDrift-r17
• NR UE: fix application of NTN TA information
• NR UE: add command line parameter ntn-initial-time-drift to compensate time drift during initial sync
• NR UE: fix writeTimestamp inconsistencies
• NR UE: add MAC state UE_RECEIVING_SIB ensuring to start RA only after successfully receiving SIBs
• rfsimulator: update earth radius for LEO simulation to match UE position in ue.conf file
• update conf files for NTN LEO scenario
• update description how to run NTN scenarios in RUNMODEM.md
• fix: ta-Common is a round-trip-time, not a one-way delay
• NR UE: simplify calculation of next_tx_slot_and_frame by moving up assignment of duration_rx_to_tx
• cellbarredNTN indicates notbarred in SIB1 if NTN access is available
• Removed b66 NTN and enabled B254 NTN conf file in CI
• NR UE: RRC layer now explicitly tells MAC layer when the RA procedure can be started
• move NTN LEO config to band 254 and update doc/RUNMODEM.md

Closes #901 (closed)