Refactor/remove base

[luke-hill]

Description

• Remove the previous Event base class and lean solely on Event::Base
• Fix all base Event namespaces from Events -> Event
• Move the default events into EventBus

Type of change

Please delete options that are not relevant.

• Refactoring (improvements to code design or tooling that don't change behaviour)

• Breaking change (will cause existing functionality to not
  work as expected)

• Please add an entry to the relevant section of CHANGELOG.md as part of this pull request.

• Also update the v19 updating_notes doc

Note to other contributors

If your change may impact future contributors, explain it here, and remember to update README.md and CONTRIBUTING.md accordingly.

Checklist:

Your PR is ready for review once the following checklist is
complete. You can also add some checks if you want to.

• Tests have been added for any changes to behaviour of the code
• New and existing tests are passing locally and on CI
• bundle exec rubocop reports no offenses
• CHANGELOG.md has been updated