Release logic misses case where first release was an alpha

[codebytere]

There's currently a bug where some PRs merged will show no release from the primary comment but correctly show releases for backport PRs and their associated commits.

For example, electron/electron#40634 was merged into main on November 30th, and subsequently backported to 28, 27, and 26. The first release of electron/electron#40634 occurred in v29.0.0-alpha.1, which can be seen in the PR's merge commit electron/electron@b2fcc15.

However, the existing source PR release logic only checks for nightlies and in the release branches specified by trop's backport comments.

We need to augment this logic to account for the first release of a given source PR commit being in a major line prerelease version and not a nightly.