[CALCITE-6290] Incorrect return type for BigQuery TRUNC #497

Job	Run time
Linux (JDK 11), Avatica main	7m 23s
CheckerFramework (JDK 11)	6m 15s
Error Prone (JDK 11), latest Guava	4m 43s
Linux (JDK 11), Pacific/Chatham Timezone	6m 27s
CheckerFramework (JDK 11), oldest Guava	6m 16s
Druid Tests	6m 21s
Linux (JDK 17)	6m 43s
Linux (JDK 19)	6m 30s
Linux (JDK 8), latest Guava, America/New_York Timezone	6m 38s
Linux (JDK 8), oldest Guava, America/New_York Timezone	6m 24s
Linux (JDK 8) Slow Tests	0s
macOS (JDK 19)	6m 36s
Windows (JDK 17)	6m 31s
Windows (JDK 8)	6m 20s
	1h 23m 7s

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