patch union_relations macro to avoid including 'collation' in cast

[colin-rogers-dbt]

resolves #

Problem

We recently updated dbt-snowflake to return the collation on a column as part of the data_type as it will soon be required when altering a column's data type (if it's a string). Most of the time this is correct, unless you want to use that datatype in a cast.

Solution

Add a macro with a snowflake specific override to scrub collation from the datatype if it's present.

Checklist

• This code is associated with an issue which has been triaged and accepted for development.
• I have read the contributing guide and understand what's expected of me
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the README.md (if applicable)

[dbeatty10]

Solved in dbt-labs/dbt-adapters#1510 instead