Elimination of Bootstrap Class

### Issue description

As we have gone through the upgrade of Grails 7, we have noticed a lot of empty / default generated files. One of Grail's core principles is convention over configuration. The default, empty Bootstrap file is a good example of not adhering to this principle. However, for functional testing, we need to make sure there is test coverage for grails supported features.

We could remove the empty Bootstrap classes, and leave them in one spot with a comment on why an empty one exists. However, after investigating the changes to the Application class, Spring has taken an alternative approach. Spring uses the application class as a bootstrap class. I can't think of a reason why we couldn't do something similar.

This ticket would remove the Bootstrap.groovy support and integrate it into the Application class instance. This is a post 7.0 goal.

Start by reviewing the existing Bootstrap.groovy support and the Application class changes described in the issue. Determine how supported features are functionally tested, then verify that Bootstrap integration is removed without losing coverage and that the application class provides the required bootstrap behavior.

Written by the indexing model from the issue text.