RATIS-2356. Define maven.compiler.release globally #304

Job	Run time
CI / release / release	4m 31s
CI / build / build	2m 46s
CI / basic (author) / author	11s
CI / basic (findbugs) / findbugs	2m 2s
CI / basic (checkstyle) / checkstyle	29s
CI / basic (rat) / rat	21s
CI / unit (misc) / unit (misc)	11m 23s
CI / unit (grpc) / unit (grpc)	15m 20s
CI / unit (flaky) / unit (flaky)	3m 34s
CI / unit (server) / unit (server)	15m 57s
CI / repro / repro	2m 44s
CI / compile (11) / compile (11)	1m 2s
CI / compile (17) / compile (17)	1m 7s
CI / compile (21) / compile (21)	1m 0s
CI / coverage	0s
	1h 2m 27s

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