[Bug 1.8.latest] Pin build dependencies to final releases

[mikealfare]

resolves #930

Problem

We never pinned build dependencies to final releases when creating the minor branch.

Solution

Pin dbt-common, dbt-adapters, and dbt-core to final releases to avoid getting pre-releases (e.g. 1.9.0b1).

Checklist

• 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
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX