Convert Entity to EntityBase for consistency with ContentEntityBase and ConfigEntityBase

Problem/Motivation

Currently we have the abstract classes Entity, ContentEntityBase and ConfigEntityBase. Both ContentEntityBase and ConfigEntityBase extend from Entity. The naming of the abstract class Entity is inconsistent with the naming of the other classes.

We use the *Base naming convention at many places - for example WidgetBase, EntityDisplayBase, EntityReferenceFormatterBase, FormatterBase and many others ...

Proposed resolution

1. Rename Entity to EntityBase.
2. Provide a class Entity in the same namespace for backwards compatibility, but flag it as deprecated.

Remaining tasks

Review.

User interface changes

API changes

Data model changes

Release notes snippet