Bump external/bpf_conformance from `514fa97` to `28ddd7f` #1498

Job	Run time
build_windows (Debug, tests)	40m 8s
build_ubuntu (Debug, tests)	21m 48s
build_windows (Debug, library)	3m 51s
build_ubuntu (Debug, library)	1m 4s
build_windows (Release, tests)	10m 47s
build_ubuntu (Release, tests)	6m 3s
build_windows (Release, library)	4m 28s
build_ubuntu (Release, library)	1m 8s
	1h 29m 17s

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