docs: `PredictTextSentimentAnalysisSample` uses unsafe JSON serialization strategy

[wallind]

Description

The PredictTextSentimentAnalysisSample Java code sample used in documentation here has a flaw as is.

If passed an input containing the " character like how much for 24"x24" pizza boxes it throws an error:

Exception in thread "main" com.google.protobuf.InvalidProtocolBufferException: com.google.gson.stream.MalformedJsonException: Unterminated object at line 1 column 49 path $.content
	at com.google.protobuf.util.JsonFormat$ParserImpl.merge(JsonFormat.java:1347)
	at com.google.protobuf.util.JsonFormat$Parser.merge(JsonFormat.java:477)
	at aiplatform.PredictTextSentimentAnalysisSample.predictTextSentimentAnalysis(PredictTextSentimentAnalysisSample.java:61)
	at aiplatform.PredictTextSentimentAnalysisSample.main(PredictTextSentimentAnalysisSample.java:39)

This can be resolved if the input is properly serialized using com.google.gson which is already included in the sample's dependencies.

Additional Context

I was referred here by @kweinmeister as I'm told this is the new house of what used to live at https://github.com/googleapis/java-aiplatform.

This issue is duplicate of one there: https://github.com/googleapis/java-aiplatform/issues/1097