Deprecate the `Bootstrapper` interface entirely and provide a
`BootstrapRegistryInitializer` alternative so that people can migrate.

Unfortunately our previous attempt to fix the typo in the `Bootstrapper`
interface didn't provide us a way to remove the deprecated method
without impacting users. It was also problematic for people who were
implementing `Bootstrapper` rather than using a lambda since they needed
to introduce the deprecated method.

We unfortunately can't see a way to fix the original typo without
introducing a new interface.

Fixes gh-25735