Compatibility for dbt v0.20, dbt-utils v0.7, spark3

[jtcohen6]

Nothing here is a breaking change:

• There are a few small tweaks to dateadd and datediff
• The addition of concat, to account for dbt_utils.default__concat switching from concat() to || in 0.7.0 (but this is backwards compatible with older versions of dbt-utils)

The integration tests update to the latest dispatch project config syntax.

Before merge

• Wait for dbt-spark==0.20.0rc1
• Wait for dbt-utils==0.7.0
• Will need to upgrade dockerized Spark version to 3 (I haven't been able to get this working), or use Databricks cluster for dbt-utils testing as well