RATIS-2490. Bump spotbugs-maven-plugin to 4.8.6.8 #683

Job	Run time
CI / release / release	5m 32s
CI / build / build	1m 48s
CI / basic (rat) / rat	22s
CI / unit (server) / unit (server)	16m 8s
CI / basic (checkstyle) / checkstyle	31s
CI / basic (author) / author	7s
CI / basic (findbugs) / findbugs	2m 27s
CI / unit (flaky) / unit (flaky)	4m 3s
CI / unit (grpc) / unit (grpc)	17m 49s
CI / unit (misc) / unit (misc)	12m 21s
CI / repro / repro	1m 35s
CI / compile (8) / compile (8)	55s
CI / compile (17) / compile (17)	1m 6s
CI / compile (11) / compile (11)	1m 2s
CI / compile (21) / compile (21)	1m 5s
CI / coverage	0s
	1h 6m 51s

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