Bump k8s.io/apimachinery from 0.23.5 to 0.28.2 in /extension/observer/k8sobserver #1425

Job	Run time
setup-environment	14m 18s
stability-test (TestStabilityMetricsOTLP)	42s
stability-test (TestStabilityMetricsOpenCensus)	1m 1s
stability-test (TestStabilityMetricsCarbon)	1m 19s
stability-test (TestStabilityMetricsSignalFx)	44s
stability-test (TestStabilityTracesOpenCensus)	1m 12s
stability-test (TestStabilityTracesSAPM)	1m 3s
stability-test (TestStabilityTracesOTLP)	1m 9s
stability-test (TestStabilityTracesJaegerGRPC)	1m 5s
stability-test (TestStabilityTracesZipkin)	44s
	23m 17s

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