Deprecate the "strict" argument in MessageType.union()

As discussed in PARQUET-379, non-strict schema merging doesn't really make any sense and we always set to true throughout the code base. Should probably deprecate it and make sure no internal code ever use non-strict schema merging.

**Reporter**: [Cheng Lian](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=lian+cheng) / @liancheng

_{**Note**: *This issue was originally created as [PARQUET-754](https://issues.apache.org/jira/browse/PARQUET-754). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Start at the MessageType.union() entry point and review the PARQUET-379 discussion and current internal call sites. Confirm that non-strict schema merging is no longer used internally, then verify that the strict argument is deprecated and update the migration documentation referenced in the issue if needed.

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