[release] prepare for 0.15.6 #315

Job	Run time
maven-test (21, oracle, jruby-9.4.8.0)	2m 22s
maven-test (11, zulu, jruby-9.4.5.0)	2m 21s
maven-test (jruby-9.4.14.0, 25, temurin)	2m 17s
maven-test (jruby-9.4.14.0, 11, temurin)	3m 3s
maven-test (8, temurin, jruby-9.2.19.0)	2m 55s
maven-test (8, temurin, jruby-9.3.3.0)	2m 29s
maven-test (11, temurin, jruby-9.2.20.1)	2m 49s
maven-test (25, temurin, jruby-10.0.3.0)	2m 30s
maven-test (jruby-9.4.14.0, 21, temurin)	2m 22s
maven-test (jruby-9.4.14.0, 8, temurin)	2m 22s
maven-test (21, corretto, jruby-head)	2m 35s
maven-test (21, oracle, jruby-9.3.13.0)	3m 4s
maven-test (24, zulu, jruby-10.0.1.0)	2m 24s
maven-test (11, temurin, jruby-9.3.13.0)	2m 25s
maven-test (21, temurin, jruby-10.0.4.0)	2m 14s
	38m 12s

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