Refactor to new CHT pipeline pattern

Closes: https://dccscr.dsop.io/dsop/gitlab/gitlab/gitaly/issues/1

• Removes gitlab versioned folders prior to 12.8
• Unwraps the 12.8 folder into the root
• Removes the prebuild.sh script in favor of download.json
• Drops the build script for now
• Update prepare script to use local downloaded artifact