Prepare for partial release.

- Default to `Rails::DeprecatedSanitizer` in `ActionView::Helpers::SanitizeHelper`.
- Add upgrade notes.
- Add sanitizer to new applications Gemfiles.
- Remove 'rails-dom-testing' as a dependency.
@kaspth do we print a deprecation message for this? would be nice if we can figure out a sensible place to do this, so that...

1. People who don't use the `sanitize` helpers at all will not see the warning
2. People who use the old default will be nagged about the switch in the next version
3. People who have the new gem installed will not see the warning

Similar to https://github.com/rails/rails/pull/16537. wdyt?