chore: Migrate Junit 4 to Junit 5 for showcase #6822

Job	Run time
build (11)	12m 24s
build (17)	12m 28s
build(21) except self-service clients	11m 13s
build(8) except for gapic-generator-java	4m 3s
build(8) for gapic-generator-java	9m 19s
compatibility	1m 42s
gapic-generator-java-bom	1m 39s
lint	24s
showcase (11)	4m 6s
showcase (17)	4m 1s
showcase-clirr	2m 59s
showcase-native	8m 17s
unmanaged_dependency_check	3m 58s
dependency_analyzer_unit_test	11s
	1h 16m 44s

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