AP_Mission: don't allow waypoint 0 (home) to be changed #45383

Job	Run time
build (stm32f7, chibios, 10)	9m 44s
build (stm32h7, chibios, 10)	8m 49s
build (fmuv2-plane, chibios, 10)	4m 4s
build (periph-build, chibios, 10)	4m 21s
build (iofirmware, chibios, 10)	1m 30s
build (CubeOrange-bootloader, chibios, 10)	54s
build (fmuv3-bootloader, chibios, 10)	1m 20s
build (revo-bootloader, chibios, 10)	54s
build (stm32h7-debug, chibios, 10)	5m 14s
build (fmuv3, chibios, 10)	12m 0s
build (revo-mini, chibios, 10)	8m 53s
build (MatekF405-Wing, chibios, 10)	9m 31s
build (CubeOrange-ODID, chibios, 10)	6m 11s
build (CubeRedPrimary-bootloader, chibios, 10)	1m 0s
build (configure-all, chibios, 10)	11m 29s
build (build-options-defaults-test, chibios, 10)	17m 36s
build (signing, chibios, 10)	5m 43s
build (CubeOrange-PPP, chibios, 10)	4m 48s
build (CubeRed-EKF2, chibios, 10)	4m 51s
build (SOHW, chibios, 10)	8m 9s
build (Pixhawk6X-PPPGW, chibios, 10)	1m 22s
build (new-check, chibios, 10)	10m 17s
	2h 18m 40s

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