Move off deprecated code in reference documentation

[daehwan2yo]

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hi,
From existing example code

• https://docs.spring.io/spring-data/cassandra/docs/current/reference/html/#cassandra.cassandra-java-config
  • Example 56

found mappingContext.setUserTypeResolver(..)which is Deprecated
so I suggest to change creating bean like this.

[mp911de]

Thank you for your contribution. That's merged, polished, and backported now.