AP_RCTelemetry: rename sensor_status_flags to unhealthy_sensors #9825

Job	Run time
build (clang)	2m 8s
build (base)	6m 38s
autotest (sitltest-plane-tests1a)	15m 51s
autotest (sitltest-plane-tests1b)	26m 33s
autotest (sitltest-quadplane)	19m 58s
	1h 11m 8s

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