Pin API versions to appropriate months.

[dbart01]

What this does

Obtains the current API version by rounding the month down to the last release instead of using the current month. Doing so let's us manually run the build and release workflow outside of version release months (at any time).

[cocoahero]

• We should probably rename Scripts/get_version to get_sdk_version to avoid confusion
• Perhaps we can look into adding an argument when the build workflow is invoked manually: https://docs.github.com/en/free-pro-team@latest/actions/reference/events-that-trigger-workflows#example-workflow-configuration