RATIS-2489. Bump exec-maven-plugin to 3.6.3 #677

Job	Run time
CI / release / release	5m 27s
CI / build / build	1m 42s
CI / unit (server) / unit (server)	16m 53s
CI / unit (grpc) / unit (grpc)	17m 48s
CI / unit (flaky) / unit (flaky)	4m 8s
CI / unit (misc) / unit (misc)	12m 26s
CI / basic (findbugs) / findbugs	2m 24s
CI / basic (checkstyle) / checkstyle	30s
CI / basic (rat) / rat	17s
CI / basic (author) / author	9s
CI / repro / repro	1m 37s
CI / compile (17) / compile (17)	1m 0s
CI / compile (8) / compile (8)	50s
CI / compile (11) / compile (11)	1m 7s
CI / compile (21) / compile (21)	1m 8s
CI / coverage	0s
	1h 7m 26s

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