Fix example code in Pipelining docs

[wlsh44]

Issue

When executing the example code from the Pipelining documentation using Spring Data Redis 4.0.0, a ClassCastException is thrown while attempting to cast RedisConnection to StringRedisConnection.

Related Docs

Example code in Pipelining docs:

List<Object> results = stringRedisTemplate.executePipelined(
  new RedisCallback<Object>() {
    @Override
    public Object doInRedis(RedisConnection connection) throws DataAccessException {
      StringRedisConnection stringRedisConn = (StringRedisConnection) connection; // exception occurs
      for (int i = 0; i < batchSize; i++) {
        stringRedisConn.rPop("myqueue");
      }
      return null;
    }
  }
);

The following exception occurs at runtime:

java.lang.ClassCastException: class jdk.proxy3.$Proxy128 cannot be cast to class org.springframework.data.redis.connection.StringRedisConnection

It appears that this issue is caused by change in StringRedisTemplate.
Starting with commit 3a930ce, StringRedisTemplate no longer overrides preProcessConnection() to return a DefaultStringRedisConnection.

Fix

Fixed the example code following the same approach as the modifications applied to the testExecutePipelinedCustomSerializer() test in commit 3a930ce.

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