chore: use Junit5 in java core #2537

Job	Run time
downstream-compatibility (google-cloud-java)	10m 50s
downstream-compatibility (java-bigtable)	5m 8s
downstream-compatibility (java-bigquery)	3m 25s
downstream-compatibility (java-bigquerystorage)	9m 27s
downstream-compatibility (java-datastore)	2m 41s
downstream-compatibility (java-firestore)	1m 50s
downstream-compatibility (java-logging)	1m 40s
downstream-compatibility (java-logging-logback)	1m 38s
downstream-compatibility (java-pubsub)	2m 15s
downstream-compatibility (java-pubsublite)	2m 22s
downstream-compatibility (java-spanner-jdbc)	2m 5s
downstream-compatibility (java-spanner)	4m 26s
downstream-compatibility (java-storage)	4m 13s
downstream-compatibility (java-storage-nio)	1m 55s
	53m 55s

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