chore: add jdk 23 job for showcase tests #9859

Job	Run time
build (11)	14m 46s
build (17)	14m 33s
build(21) except self-service clients	13m 25s
build(8) except for gapic-generator-java	4m 34s
build(8) for gapic-generator-java	11m 43s
compatibility	1m 52s
dependency_analyzer_unit_test	23s
gapic-generator-java-bom	1m 53s
lint	27s
showcase (11)	4m 39s
showcase-clirr	3m 7s
showcase (17)	4m 36s
unmanaged_dependency_check	4m 28s
showcase (21)	4m 39s
showcase (23)	4m 26s
	1h 29m 31s

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