Update retrofit_gki.sh for new init_boot plan

Only GKI kernel, boot.img, need to be officially certified.
Also remove V3 & V4 retrofit plan as the latest consensus is to support
V2 only.

Bug: 210367929
Test: retrofit_gki_test
Change-Id: Ia1d81b3382fe62a39a028a933a1f4275977d8bbd
3 files changed