Update BCK bucket upload to properly check for latest release and do prerelease upload

[rtibbles]

Observed behavior

Currently the BCK bucket upload never runs because of a check against a non-existent attribute of the release event payload.

Expected behavior

Upload prereleases to a prerelease bucket (under the secret BCK_PRERELEASE_BUILD_ARTIFACT_GCS_BUCKET) for early warnings of deployment issues.

Update the final release check following the guidelines here for determining if a release is the latest release or not: https://github.com/orgs/community/discussions/64734

For all prereleases, we should also upload hashi assets to the bucket stored at this secret: STUDIO_BCK_CONTENT_STATIC_BUCKET