[CT-2047] Dynamically list all .latest branches for scheduled testing

[emmyoop]

Related to dbt-labs/dbt-core#5773

Can copy the implementation for dbt-core here

When we cut the 1.5.latest branch, instead of just adding 1.5.latest to the release test, update it with the above logic to auto generate the list of branches to test. Eventually we will need logic to "start from" as we remove support for older versions. We should do that centrally in the action since all adapters support the same versions.