Update Trestle demo template to use Trestle-bot

[beatrizmcouto]

Currently bash automation is being used and the team needs to update to Trestle-bot

Update this repo - https://github.com/RedHatProductSecurity/trestle-demo. To use the trestle-bot project instead of the legacy automation approach to allow developers of trestle-bot to easily test predefined workflows with GitHub Actions.

An example repository (for GitHub Actions workflows) is here - https://github.com/jpower432/cautious-potato/tree/main

Update Trestle demo template to use Trestle-bot
Git sub-tree is removed (vendor dir and references to it)