pr_body context: use a tera template for the default configuration

[MarcoIeni]

#1778 added the pr_body context.
However it maintained the old logic to construct the default pr_body:

https://github.com/MarcoIeni/release-plz/blob/main/crates/release_plz_core/src/pr.rs#L134

This is totally fine because I like small PRs 👍

However, it would be cool to use a tera template to render the template of the PR body.
In this way, we could easily document what's the default pr_body, so that people can create their own template more easily