[CT-3084] [Bug] UnparsedVersion.__lt__ is sensitive to order

[MichelleArk]

The current implementation of UnparsedVersion.__lt__ is sensitive to the order of arguments, leading to surprising behaviour such as:

>>> from dbt.contracts.graph.unparsed import UnparsedVersion
>>> UnparsedVersion(12) < UnparsedVersion("2")
True
>>> UnparsedVersion(12) > UnparsedVersion("2")
True

This method also powers the version selection method and can lead to issues there. For example, if a model has versions [1, "2", 12], and when "2" is the latest version, version 12 is both an old and prerelease version.