Update README to indicate Java 21 is now default instead of Java 17 #345

Job	Run time
ci (jdk11, 11)	57s
ci (jdk11-alpine, 11, alpine)	40s
ci (jdk11-focal, 11, focal)	1m 3s
ci (jdk17, 17)	50s
ci (jdk17-alpine, 17, alpine)	39s
ci (jdk17-focal, 17, focal)	54s
ci (jdk17-graal, 17, graal)	1m 30s
ci (jdk23, 23)	53s
ci (jdk23-alpine, 23, alpine)	38s
ci (jdk23-graal, 23, graal)	1m 53s
ci (jdk17-focal-graal, 17, focal-graal)	1m 15s
ci (jdk21, 21)	54s
ci (jdk21-alpine, 21, alpine)	41s
ci (jdk21-graal, 21, graal)	1m 35s
ci (jdk-lts-and-current, 21, 23)	59s
ci (jdk-lts-and-current-alpine, 21, 23, alpine)	44s
ci (jdk-lts-and-current-graal, 21, 23, graal)	1m 42s
	17m 47s

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