Fix prepare_release.py based on the changes made in Citus repo

[onurctirtir]

The following changes were made in Citus repo in the past two releases:

• config.py is moved into a different directory
• confifure.in is renamed to configure.ac
• MASTER_VERSION string constant now uses double quotes instead of single quotes
• Development branch is renamed to "main"
• multi_extension.sql now needs to call some utility functions and needs to drop the temporary tables by qualifying their schemas