[Go][Packaging] Revisit the license file attachment to the go packages

As per https://github.com/apache/arrow/pull/7355#issuecomment-639560475

A nicer solution would be to rename the top level LICENSE.txt to LICENSE, so we wouldn't need to maintain another copy of it.

**Reporter**: [Krisztian Szucs](https://issues.apache.org/jira/browse/ARROW-9044) / @kszucs

_{**Note**: *This issue was originally created as [ARROW-9044](https://issues.apache.org/jira/browse/ARROW-9044). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}