KAFKA-16934: Clean up and refactor release.py

[soarez]

The current release script has a couple of issues:

• It's a single long file with duplicated logic, which makes it difficult to understand and make changes
• When a command fails, the user is forced to start from the beginning, expanding feedback loops. e.g. publishing step fails because the credentials were set incorrectly in ~/.gradle/gradle.properties

This change moves release.py and release_notes.py into new components under release/.