ci: add Java 21 to CI matrix #3247

Job	Run time
Examples / MigrationExamples	1m 8s
Static_Analysis / SpotBugs	17s
Build (17, temurin) / Build	9m 55s
Build (17, corretto) / Build	9m 37s
Build (8, corretto) / Build	12m 40s
Build (11, corretto) / Build	10m 37s
Build (21, corretto) / Build	9m 40s
Build (8, temurin) / Build	8m 41s
Build (21, temurin) / Build	4m 20s
Build (11, temurin) / Build	15m 19s
notify	-1s
	1h 22m 13s

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