Remove @internal from ContentEntityDeleteForm generic base class

Problem/Motivation

Contrib and custom content entity types need a way to just declare a standard content entity form delete handler. (Or need a way to extend a convenient base form if overrides are needed.)

Such a base class exists in core, even if its name doesn't end with "Base". The short description is "Provides a generic base class for a content entity deletion form."
Unfortunately it was declared an internal base class in #2873696: Add @internal to core form classes, methods, and properties. I think this might have been an oversight. Or a case of better safe than sorry. Exposing stuff (again) is much easier than marking things internal once it's out there.
It is extremely convenient to have a class that provides the standard content entity form deletion behavior.
However, the class being internal makes this problematic for contrib.

Examples module extends ContentEntityConfirmFormBase for its non-translatable content entity type. For translatable entity types things become difficult. I have no idea what else to do than copying ContentEntityDeleteForm, but that's a maintenance nightmare. No wonder contrib is just ignoring the @internal tag. Still a maintenance issue, but it seems the worst that could happen is disappearing delete functionality. Much better than buggy delete functionality and possible data loss.

For non-translatable entity types it's doable to extend ContentEntityConfirmFormBase, but this is still more work than just declaring ContentEntityDeleteForm if you just need a standard entity form delete handler. And extending ContentEntityConfirmFormBase could be risky if the entity type would one day be converted to be translatable. ContentEntityDeleteForm handles both cases since #2486177: Deleting an entity translation from the UI deletes the whole entity.

So unless I'm missing something, it appears to be problematic for contrib and custom code that ContentEntityDeleteForm is declared @internal.

I've looked at #2491057: Streamline the entity deletion form class hierarchy. Doesn't seem it would help. And doesn't look like a blocker.

Steps to reproduce

Proposed resolution

Remove @internal from the class.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet