RATIS-2490. Bump spotbugs-maven-plugin to 4.8.6.8 (#1421) #684

Job	Run time
CI / build / build	1m 57s
CI / unit (server) / unit (server)	18m 18s
CI / unit (grpc) / unit (grpc)	19m 3s
CI / unit (misc) / unit (misc)	13m 28s
CI / release / release	5m 16s
CI / basic (author) / author	10s
CI / unit (flaky) / unit (flaky)	4m 45s
CI / basic (rat) / rat	19s
CI / basic (checkstyle) / checkstyle	26s
CI / basic (findbugs) / findbugs	2m 29s
CI / repro / repro	1m 35s
CI / compile (11) / compile (11)	1m 4s
CI / compile (8) / compile (8)	57s
CI / compile (17) / compile (17)	1m 3s
CI / compile (21) / compile (21)	1m 6s
CI / coverage	3m 32s
	1h 15m 28s

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