update vertex ai sample to use gemini 2.0 flash #922

Job	Run time
spm-package-resolved	1m 11s
pod-lib-lint (ios, macos-14)	8m 53s
sample (macos-13, Xcode_15.2)	5m 53s
sample (macos-14, Xcode_15.4)	1m 45s
spm-unit (macos-13, Xcode_15.2, iOS)	4m 33s
sample (macos-15, Xcode_16.2)	2m 33s
spm-unit (macos-14, Xcode_15.4, iOS)	3m 8s
testapp-integration (iOS, macos-15)	3m 35s
spm-unit (macos-15, Xcode_16.2, iOS)	2m 13s
spm-unit (macos-15, Xcode_16.2, tvOS)	1m 58s
spm-unit (macos-15, Xcode_16.2, macOS)	1m 49s
spm-unit (macos-15, Xcode_16.2, watchOS)	3m 8s
spm-unit (macos-15, Xcode_16.2, catalyst)	2m 56s
spm-unit (macos-15, Xcode_16.2, visionOS)	2m 26s
	46m 1s

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