[CT-2032] Make fetch-repo-branches smart enough to only return what we support

Update (fetch-repo-branches)[https://github.com/dbt-labs/actions/tree/main/fetch-repo-branches] to only return the `*.latest` branches that we support, once we start dropping support of older branches. Possibly update via a parameter?

Start by inspecting the fetch-repo-branches action at https://github.com/dbt-labs/actions/tree/main/fetch-repo-branches and identify how it currently selects *.latest branches. Clarify which branches remain supported and whether that list should be configurable; done means the action returns only supported branches after older branch support is dropped.

Written by the indexing model from the issue text.