Update migration guide

The built-in Hive 2.3 fixes the following issues:

• HIVE-6727: Table level stats for external tables are set incorrectly.
• HIVE-15653: Some ALTER TABLE commands drop table stats.
• SPARK-12014: Spark SQL query containing semicolon is broken in Beeline.
• SPARK-25193: insert overwrite doesn't throw exception when drop old data fails.
• SPARK-25919: Date value corrupts when tables are "ParquetHiveSerDe" formatted and target table is Partitioned.
• SPARK-26332: Spark sql write orc table on viewFS throws exception.
• SPARK-26437: Decimal data becomes bigint to query, unable to query.

We need update migration guide.