chore: demo new Timestamp test usign threeten in java-core #8951

Job	Run time
build (11)	15m 20s
build (17)	14m 34s
build(21) except self-service clients	13m 50s
build(8) except for gapic-generator-java	4m 25s
build(8) for gapic-generator-java	11m 57s
compatibility	1m 29s
lint	20s
dependency_analyzer_unit_test	12s
gapic-generator-java-bom	1m 48s
showcase (11)	3m 58s
showcase-clirr	3m 8s
showcase (17)	3m 54s
showcase (21)	3m 56s
showcase-native	8m 1s
unmanaged_dependency_check	3m 51s
	1h 30m 43s

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